diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..ee46411
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+/dist/
+*.exe
diff --git a/BUILD.md b/BUILD.md
new file mode 100644
index 0000000..879ba8d
--- /dev/null
+++ b/BUILD.md
@@ -0,0 +1,276 @@
+# Building and developing FancyWin
+
+This document covers source builds, cross-compilation, verification, release
+artifacts, dependencies, and the Windows implementation. End-user instructions
+are in [README.md](README.md).
+
+## Requirements
+
+- Go 1.24 or later
+- PowerShell for the supplied Windows build script, or a POSIX shell for manual
+  cross-compilation
+- Windows 11 for runtime integration testing
+
+The project uses pure Go and sets `CGO_ENABLED=0`. No C compiler, Windows SDK,
+.NET SDK, Visual Studio, or PowerToys checkout is required.
+
+## Repository layout
+
+```text
+cmd/fancywin/                          CLI entry point
+internal/config/                       YAML model, loading, and validation
+internal/layout/                       Platform-neutral zone geometry
+internal/platform/platform_windows.go  Win32 backend
+internal/platform/platform_other.go    Non-Windows diagnostic stub
+fancywin.example.yaml                  Example user configuration
+build.ps1                              Native Windows build script
+dist/                                  Generated portable artifacts
+```
+
+## Dependencies
+
+Runtime functionality uses the Windows system DLLs available with Windows 11.
+There are two pinned Go module dependencies:
+
+- `golang.org/x/sys/windows` for Windows handles, callbacks, process queries,
+  and UTF-16 helpers
+- `gopkg.in/yaml.v3` for strict YAML parsing
+
+Both modules are statically linked into the executable. Exact versions and
+checksums are recorded in `go.mod` and `go.sum`.
+
+## Native Windows build
+
+From PowerShell in the repository root:
+
+```powershell
+.\build.ps1
+```
+
+The script:
+
+1. Selects `amd64` or `arm64` from `PROCESSOR_ARCHITECTURE`.
+2. Sets `GOOS=windows` and disables CGO.
+3. Runs the complete Go test suite.
+4. Builds a stripped, reproducible-path executable.
+5. Writes `dist\fancywin.exe` and copies the example to
+   `dist\fancywin.yaml`.
+
+The generated executable is a console subsystem application so users can see
+configuration and Win32 errors directly.
+
+## Cross-compiling
+
+Build Windows x64 from Linux, macOS, or another Go-supported host:
+
+```sh
+mkdir -p dist
+CGO_ENABLED=0 GOOS=windows GOARCH=amd64 go build \
+  -buildvcs=false \
+  -trimpath \
+  -ldflags='-s -w' \
+  -o dist/fancywin.exe \
+  ./cmd/fancywin
+cp fancywin.example.yaml dist/fancywin.yaml
+```
+
+For Windows on ARM:
+
+```sh
+CGO_ENABLED=0 GOOS=windows GOARCH=arm64 go build \
+  -buildvcs=false \
+  -trimpath \
+  -ldflags='-s -w' \
+  -o dist/fancywin-arm64.exe \
+  ./cmd/fancywin
+```
+
+`-trimpath` removes local source paths. `-s -w` removes symbol and DWARF tables
+to reduce the portable binary size. `-buildvcs=false` permits builds from source
+archives or workspaces without complete Git metadata.
+
+## Verification
+
+Run platform-neutral tests and race detection on the development host:
+
+```sh
+go test -buildvcs=false ./...
+go test -buildvcs=false -race ./...
+```
+
+Validate Windows-only code without executing it:
+
+```sh
+GOOS=windows GOARCH=amd64 CGO_ENABLED=0 go vet -buildvcs=false ./...
+GOOS=windows GOARCH=amd64 CGO_ENABLED=0 go build -buildvcs=false ./cmd/fancywin
+GOOS=windows GOARCH=arm64 CGO_ENABLED=0 go build -buildvcs=false ./cmd/fancywin
+```
+
+Validate the example configuration on any supported development host:
+
+```sh
+go run -buildvcs=false ./cmd/fancywin \
+  -config fancywin.example.yaml \
+  -check
+```
+
+Cross-compilation proves that the Windows API bindings type-check; it does not
+replace runtime testing on a Windows desktop.
+
+## Windows smoke-test checklist
+
+Test both x64 and ARM64 where hardware is available:
+
+1. Start FancyWin without elevation and confirm the monitor names and work areas
+   are printed.
+2. Confirm a second instance exits with a single-instance error.
+3. Shift-drag Win32, UWP/Windows App SDK, and Chromium-based windows into each
+   zone.
+4. Exercise all configured hotkey actions, including wrap-around.
+5. Test monitors with negative virtual-screen coordinates, different DPI
+   scaling, portrait orientation, and taskbars on different edges.
+6. Maximize a window and then move it by hotkey, checking visible-frame
+   alignment.
+7. Edit valid YAML and confirm it reloads within roughly two seconds.
+8. Introduce invalid YAML and confirm the previous configuration stays active.
+9. Verify exact and substring exclusions.
+10. Confirm a normal process cannot move an elevated application and that a
+    same-integrity elevated run can do so.
+11. Disconnect and reconnect a monitor and confirm current work-area information
+    is used for subsequent moves.
+
+## Architecture
+
+### Process model
+
+FancyWin is a single process and a single portable executable. The main
+goroutine is locked to its OS thread because Windows delivers the out-of-context
+accessibility callback and thread hotkey messages through that thread's message
+queue. The program does not inject code into other processes or install a hook
+DLL, service, shell extension, or driver.
+
+At startup the backend:
+
+1. Requests per-monitor-v2 DPI awareness.
+2. Creates a named mutex for single-instance enforcement.
+3. Installs a `SetWinEventHook` covering `EVENT_SYSTEM_MOVESIZESTART` through
+   `EVENT_SYSTEM_MOVESIZEEND` with `WINEVENT_OUTOFCONTEXT` and
+   `WINEVENT_SKIPOWNPROCESS`.
+4. Registers configured system-wide hotkeys against the current thread.
+5. Creates a two-second timer for configuration reload checks and a 25 ms timer
+   for sampling the mouse button, Shift, and active drag state.
+6. Enters a standard Win32 `GetMessage` loop.
+
+### Mouse snapping
+
+The move-start callback records a manageable top-level window. On move-end, the
+backend uses the most recently sampled asynchronous Shift-key state, obtains the
+pointer's monitor, resolves the applicable layout, and finds the first zone
+containing the pointer. Sampling throughout the drag avoids losing activation
+when Windows delivers the move-end accessibility event just after a key-state
+transition. The backend then converts that percentage zone into the monitor work
+area's physical coordinates and calls `SetWindowPos`.
+
+The configured gap affects the destination rectangle but not zone hit testing.
+This avoids dead pointer strips between adjacent zones.
+
+Some Windows environments do not deliver the move/size accessibility events
+reliably. The 25 ms input timer provides an independent fallback: at the initial
+left-button press it records the manageable top-level window beneath the
+pointer, observes whether that window's rectangle actually changes while the
+button remains down, tracks Shift, and snaps on release. Ordinary clicks are
+discarded because the recorded rectangle did not move. Coordination state
+prevents the accessibility and polling paths from snapping the same drag twice.
+
+### Keyboard snapping
+
+`RegisterHotKey` posts `WM_HOTKEY` to the message-loop thread. The backend obtains
+the foreground window and its nearest monitor. Direct `zone_N` actions use a
+one-based index. Previous/next actions find the zone whose center is closest to
+the current window center and wrap through the monitor's ordered zone list.
+
+`MOD_NOREPEAT` prevents a held shortcut from generating repeated moves.
+
+### Coordinates and visible frames
+
+Zone percentages are resolved against `MONITORINFOEX.rcWork`, not the complete
+monitor rectangle, so taskbar and app-bar space is excluded. Per-monitor-v2 DPI
+awareness keeps pointer, monitor, and window coordinates in one physical-pixel
+coordinate system across mixed-DPI displays.
+
+Windows 11 commonly includes an invisible resize border in `GetWindowRect`.
+Before moving a window, FancyWin compares that rectangle with
+`DWMWA_EXTENDED_FRAME_BOUNDS` and expands the outer `SetWindowPos` rectangle so
+the visible frame aligns with the requested zone.
+
+### Overlay rendering
+
+The overlay is implemented directly with User32 and GDI. While an active mouse
+drag and Shift are both detected, FancyWin creates one borderless popup window
+covering the usable work area of the monitor beneath the pointer. The window
+uses `WS_EX_LAYERED`, `WS_EX_TRANSPARENT`, `WS_EX_TOOLWINDOW`,
+`WS_EX_NOACTIVATE`, and `WS_EX_TOPMOST` so it remains visible without receiving
+input, taking focus, or appearing in Alt+Tab.
+
+The background is painted with a color key made transparent by
+`SetLayeredWindowAttributes`. GDI then draws the gap-adjusted zone rectangles
+and centered Segoe UI labels in the configured color and opacity. The overlay is
+destroyed when Shift or the left mouse button is released. Moving the pointer to
+another monitor recreates it with that monitor's work area and selected layout.
+
+### Window filtering and permissions
+
+The backend ignores invisible windows, child windows, owned windows, and
+configured executable exclusions. Process names are obtained with
+`PROCESS_QUERY_LIMITED_INFORMATION` and `QueryFullProcessImageName`.
+
+Windows User Interface Privilege Isolation prevents a lower-integrity process
+from reliably controlling a higher-integrity window. FancyWin does not attempt
+to bypass that boundary or request elevation through a manifest.
+
+### Configuration reload
+
+The YAML decoder rejects unknown fields. Semantic validation checks the schema
+version, gap range, unique monitor entries, zone bounds, hotkey action syntax,
+and required values.
+
+The message-loop timer checks the configuration modification time. A valid new
+configuration replaces the active one and re-registers hotkeys. If parsing,
+validation, or hotkey registration fails, the previous valid configuration and
+hotkeys are restored.
+
+## Current scope
+
+The current backend intentionally excludes:
+
+- A visual layout editor
+- Low-level keyboard interception of Windows-reserved snap shortcuts
+- Multi-zone selection and expansion
+- Moving windows across monitors as part of next/previous traversal
+- Persistence of application-to-zone history
+- Automatic placement of newly created windows
+- Virtual-desktop-specific layouts
+- Layout switching shortcuts
+- Rounded-corner control
+
+These should be treated as separate features because several require additional
+state, UI surfaces, or lower-level input handling.
+
+## Release preparation
+
+For a distributable release:
+
+1. Run all tests, vet, both architecture builds, and the Windows smoke-test
+   checklist.
+2. Update the `version` constant in `cmd/fancywin/main.go`.
+3. Build x64 and ARM64 artifacts with `CGO_ENABLED=0`, `-trimpath`, and
+   `-ldflags='-s -w'`.
+4. Include an example renamed to `fancywin.yaml` beside each executable.
+5. Record SHA-256 hashes with `Get-FileHash` or `sha256sum`.
+6. Code-sign public binaries if a trusted signing certificate and release
+   process are available. Signing is not required for portability, but unsigned
+   downloads may receive stronger Microsoft Defender SmartScreen warnings.
+
+PowerToys was used as behavioral and architectural reference material only.
+FancyWin is an independent implementation and contains no copied PowerToys
+source code.
diff --git a/README.md b/README.md
index b4d1057..a77bde9 100644
--- a/README.md
+++ b/README.md
@@ -10,6 +10,8 @@ menu entry, or registry settings.
 ## What FancyWin does
 
 - Snaps a dragged window into the zone beneath the mouse pointer.
+- Shows a transparent, click-through guide with every zone's outline and name
+  while Shift-dragging.
 - Moves the focused window between zones with keyboard shortcuts.
 - Supports different layouts for different monitors.
 - Uses percentages, so layouts adapt to resolution, orientation, display
@@ -17,10 +19,10 @@ menu entry, or registry settings.
 - Reloads its configuration automatically after the YAML file changes.
 - Lets you exclude applications that should never be moved.
 
-FancyWin is a window-moving backend rather than a complete PowerToys
-replacement. It does not display a zone overlay or include a visual layout
-editor. It also does not currently remember application positions, combine
-multiple zones, or assign layouts per virtual desktop.
+FancyWin is a focused window manager rather than a complete PowerToys
+replacement. It does not include a visual layout editor and does not currently
+remember application positions, combine multiple zones, or assign layouts per
+virtual desktop.
 
 ## Getting started
 
@@ -78,6 +80,12 @@ version: 1
 gap: 8
 shift_drag: true
 
+overlay:
+  enabled: true
+  color: "#00AEEF"
+  opacity: 90
+  border_width: 3
+
 layouts:
   - monitor: "*"
     zones:
@@ -118,10 +126,35 @@ use.
 | `version` | Configuration format. This must currently be `1`. |
 | `gap` | Inward spacing, in pixels, applied to every edge of every zone. Valid range: `0`–`500`. |
 | `shift_drag` | Enables or disables mouse snapping when Shift is held at the end of a drag. |
+| `overlay` | Controls the transparent zone guide shown during Shift-dragging. |
 | `layouts` | One or more monitor layouts containing zones. |
 | `excluded_apps` | Applications FancyWin must not move. May be empty. |
 | `hotkeys` | Global keyboard shortcuts. May be empty. |
 
+### Zone overlay
+
+The overlay appears after a window has begun moving and Shift is held. It shows
+all zones on the monitor beneath the pointer, using the same gap-adjusted
+rectangles that windows snap into. Each outline contains the zone's `name`, or
+`Zone N` when its name is empty. Releasing Shift or the mouse button immediately
+hides it.
+
+```yaml
+overlay:
+  enabled: true
+  color: "#00AEEF"
+  opacity: 90
+  border_width: 3
+```
+
+- `enabled` turns the guide on or off.
+- `color` is an outline and label color in `#RRGGBB` format.
+- `opacity` is from `1` to `100` percent.
+- `border_width` is from `1` to `20` physical pixels.
+
+The guide is transparent between its outlines, does not receive mouse input,
+does not take keyboard focus, and does not appear in Alt+Tab.
+
 ### Zone coordinates
 
 Each zone is a rectangle described as percentages of the monitor's usable work
@@ -296,6 +329,14 @@ borders, but it cannot override size rules imposed by the application itself.
 Wait at least two seconds and check the console. FancyWin keeps using the last
 valid configuration when a new edit cannot be parsed or validated.
 
+### The zone overlay does not appear
+
+- Confirm `overlay.enabled` and `shift_drag` are both `true`.
+- Begin moving the window before expecting the guide; a stationary Shift-click
+  is intentionally ignored.
+- Keep Shift held during the drag.
+- Check the console for `cannot show zone overlay` errors.
+
 ## Project relationship
 
 FancyWin is an independent implementation and does not contain Microsoft
diff --git a/build.ps1 b/build.ps1
new file mode 100644
index 0000000..ffd179f
--- /dev/null
+++ b/build.ps1
@@ -0,0 +1,11 @@
+$ErrorActionPreference = "Stop"
+$env:CGO_ENABLED = "0"
+$env:GOOS = "windows"
+$env:GOARCH = if ($env:PROCESSOR_ARCHITECTURE -eq "ARM64") { "arm64" } else { "amd64" }
+
+New-Item -ItemType Directory -Force -Path dist | Out-Null
+go test -buildvcs=false ./...
+go build -buildvcs=false -trimpath -ldflags="-s -w" -o dist/fancywin.exe ./cmd/fancywin
+Copy-Item fancywin.example.yaml dist/fancywin.yaml
+
+Write-Host "Built dist/fancywin.exe for windows/$env:GOARCH"
diff --git a/cmd/fancywin/main.go b/cmd/fancywin/main.go
new file mode 100644
index 0000000..4e84a94
--- /dev/null
+++ b/cmd/fancywin/main.go
@@ -0,0 +1,54 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/stevec/fancywin/internal/config"
+	"github.com/stevec/fancywin/internal/platform"
+	"gopkg.in/yaml.v3"
+)
+
+const version = "0.2.1"
+
+func main() {
+	exe, err := os.Executable()
+	if err != nil {
+		fatal(err)
+	}
+	configPath := flag.String("config", config.DefaultPath(exe), "path to YAML configuration")
+	check := flag.Bool("check", false, "validate configuration and exit")
+	printDefault := flag.Bool("print-default", false, "print a default configuration and exit")
+	showVersion := flag.Bool("version", false, "print version and exit")
+	debug := flag.Bool("debug", false, "log window move and snap diagnostics")
+	flag.Parse()
+
+	if *showVersion {
+		fmt.Println("fancywin", version)
+		return
+	}
+	if *printDefault {
+		b, err := yaml.Marshal(config.Default())
+		if err != nil {
+			fatal(err)
+		}
+		_, _ = os.Stdout.Write(b)
+		return
+	}
+	cfg, err := config.Load(*configPath)
+	if err != nil {
+		fatal(fmt.Errorf("%s: %w", filepath.Clean(*configPath), err))
+	}
+	if *check {
+		fmt.Printf("configuration OK: %s\n", filepath.Clean(*configPath))
+		return
+	}
+	fmt.Printf("fancywin %s using %s\n", version, filepath.Clean(*configPath))
+	if err := platform.Run(*configPath, cfg, *debug); err != nil {
+		fatal(err)
+	}
+}
+
+func fatal(err error) { fmt.Fprintln(os.Stderr, "fancywin:", err); os.Exit(1) }
diff --git a/fancywin.example.yaml b/fancywin.example.yaml
new file mode 100644
index 0000000..65f9793
--- /dev/null
+++ b/fancywin.example.yaml
@@ -0,0 +1,49 @@
+# FancyWin configuration. Percentages are relative to each monitor's usable
+# work area, so the Windows taskbar is automatically excluded.
+version: 1
+gap: 8
+shift_drag: true
+
+overlay:
+  enabled: true
+  color: "#00AEEF"
+  opacity: 90
+  border_width: 3
+
+layouts:
+  # The wildcard is used for any monitor without a more specific entry.
+  - monitor: "*"
+    zones:
+      - name: left
+        x: 0
+        y: 0
+        width: 25
+        height: 100
+      - name: middle
+        x: 25
+        y: 0
+        width: 50
+        height: 100
+      - name: right
+        x: 75
+        y: 0
+        width: 25
+        height: 100
+
+  # Optional per-display override. Get the name from Windows Display Settings
+  # or the startup diagnostics, then uncomment and edit this block.
+  # - monitor: '\\.\DISPLAY2'
+  #   zones:
+  #     - { name: main, x: 0, y: 0, width: 70, height: 100 }
+  #     - { name: side, x: 70, y: 0, width: 30, height: 100 }
+
+excluded_apps:
+  - mstsc.exe
+  - games
+
+hotkeys:
+  - { keys: "win+alt+left", action: previous_zone }
+  - { keys: "win+alt+right", action: next_zone }
+  - { keys: "win+alt+1", action: zone_1 }
+  - { keys: "win+alt+2", action: zone_2 }
+  - { keys: "win+alt+3", action: zone_3 }
diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..4eeb5dd
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,8 @@
+module github.com/stevec/fancywin
+
+go 1.24
+
+require (
+	golang.org/x/sys v0.35.0
+	gopkg.in/yaml.v3 v3.0.1
+)
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..f9193bf
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,6 @@
+golang.org/x/sys v0.35.0 h1:vz1N37gP5bs89s7He8XuIYXpyY0+QlsKmzipCbUtyxI=
+golang.org/x/sys v0.35.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/internal/config/config.go b/internal/config/config.go
new file mode 100644
index 0000000..40280ba
--- /dev/null
+++ b/internal/config/config.go
@@ -0,0 +1,179 @@
+package config
+
+import (
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+
+	"gopkg.in/yaml.v3"
+)
+
+type Config struct {
+	Version   int      `yaml:"version"`
+	Gap       int      `yaml:"gap"`
+	ShiftDrag bool     `yaml:"shift_drag"`
+	Overlay   Overlay  `yaml:"overlay"`
+	Layouts   []Layout `yaml:"layouts"`
+	Excluded  []string `yaml:"excluded_apps"`
+	Hotkeys   []Hotkey `yaml:"hotkeys"`
+}
+
+type Overlay struct {
+	Enabled     bool   `yaml:"enabled"`
+	Color       string `yaml:"color"`
+	Opacity     int    `yaml:"opacity"`
+	BorderWidth int    `yaml:"border_width"`
+}
+
+type Layout struct {
+	Monitor string `yaml:"monitor"`
+	Zones   []Zone `yaml:"zones"`
+}
+
+type Zone struct {
+	Name   string  `yaml:"name"`
+	X      float64 `yaml:"x"`
+	Y      float64 `yaml:"y"`
+	Width  float64 `yaml:"width"`
+	Height float64 `yaml:"height"`
+}
+
+type Hotkey struct {
+	Keys   string `yaml:"keys"`
+	Action string `yaml:"action"`
+}
+
+func Default() Config {
+	return Config{
+		Version: 1, Gap: 8, ShiftDrag: true,
+		Overlay: Overlay{Enabled: true, Color: "#00AEEF", Opacity: 90, BorderWidth: 3},
+		Layouts: []Layout{{Monitor: "*", Zones: []Zone{
+			{Name: "left", X: 0, Y: 0, Width: 50, Height: 100},
+			{Name: "right", X: 50, Y: 0, Width: 50, Height: 100},
+		}}},
+		Hotkeys: []Hotkey{
+			{Keys: "win+alt+left", Action: "previous_zone"},
+			{Keys: "win+alt+right", Action: "next_zone"},
+		},
+	}
+}
+
+func Load(path string) (Config, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return Config{}, err
+	}
+	defer f.Close()
+	var c Config
+	d := yaml.NewDecoder(f)
+	d.KnownFields(true)
+	if err := d.Decode(&c); err != nil {
+		return Config{}, fmt.Errorf("parse YAML: %w", err)
+	}
+	if err := c.Validate(); err != nil {
+		return Config{}, err
+	}
+	return c, nil
+}
+
+func (c Config) Validate() error {
+	if c.Version != 1 {
+		return fmt.Errorf("version must be 1 (got %d)", c.Version)
+	}
+	if c.Gap < 0 || c.Gap > 500 {
+		return errors.New("gap must be between 0 and 500 pixels")
+	}
+	if c.Overlay.Enabled {
+		if !validHexColor(c.Overlay.Color) {
+			return errors.New("overlay.color must use #RRGGBB format")
+		}
+		if c.Overlay.Opacity < 1 || c.Overlay.Opacity > 100 {
+			return errors.New("overlay.opacity must be between 1 and 100")
+		}
+		if c.Overlay.BorderWidth < 1 || c.Overlay.BorderWidth > 20 {
+			return errors.New("overlay.border_width must be between 1 and 20 pixels")
+		}
+	}
+	if len(c.Layouts) == 0 {
+		return errors.New("at least one layout is required")
+	}
+	seenMonitors := map[string]bool{}
+	for li, l := range c.Layouts {
+		name := strings.TrimSpace(l.Monitor)
+		if name == "" {
+			return fmt.Errorf("layouts[%d].monitor is required", li)
+		}
+		key := strings.ToLower(name)
+		if seenMonitors[key] {
+			return fmt.Errorf("duplicate layout for monitor %q", name)
+		}
+		seenMonitors[key] = true
+		if len(l.Zones) == 0 {
+			return fmt.Errorf("layout %q needs at least one zone", name)
+		}
+		for zi, z := range l.Zones {
+			if z.Width <= 0 || z.Height <= 0 || z.X < 0 || z.Y < 0 || z.X+z.Width > 100.000001 || z.Y+z.Height > 100.000001 {
+				return fmt.Errorf("layout %q zone %d must fit within the 0..100 percent work area", name, zi+1)
+			}
+		}
+	}
+	for i, h := range c.Hotkeys {
+		a := strings.ToLower(strings.TrimSpace(h.Action))
+		if a != "next_zone" && a != "previous_zone" && !strings.HasPrefix(a, "zone_") {
+			return fmt.Errorf("hotkeys[%d].action must be next_zone, previous_zone, or zone_N", i)
+		}
+		if strings.HasPrefix(a, "zone_") {
+			n, err := strconv.Atoi(strings.TrimPrefix(a, "zone_"))
+			if err != nil || n < 1 {
+				return fmt.Errorf("hotkeys[%d].action must use a positive zone number", i)
+			}
+		}
+		if strings.TrimSpace(h.Keys) == "" {
+			return fmt.Errorf("hotkeys[%d].keys is required", i)
+		}
+	}
+	return nil
+}
+
+func validHexColor(s string) bool {
+	if len(s) != 7 || s[0] != '#' {
+		return false
+	}
+	_, err := strconv.ParseUint(s[1:], 16, 24)
+	return err == nil
+}
+
+func DefaultPath(exePath string) string {
+	return filepath.Join(filepath.Dir(exePath), "fancywin.yaml")
+}
+
+func (c Config) LayoutFor(device string) (Layout, bool) {
+	for _, l := range c.Layouts {
+		monitor := strings.TrimSpace(l.Monitor)
+		if monitor != "*" && strings.EqualFold(monitor, strings.TrimSpace(device)) {
+			return l, true
+		}
+	}
+	for _, l := range c.Layouts {
+		if strings.TrimSpace(l.Monitor) == "*" {
+			return l, true
+		}
+	}
+	return Layout{}, false
+}
+
+func (c Config) IsExcluded(exe string) bool {
+	// Windows paths may be validated on another OS during cross-compilation.
+	normalized := strings.ReplaceAll(exe, `\`, "/")
+	base := strings.ToLower(normalized[strings.LastIndex(normalized, "/")+1:])
+	for _, item := range c.Excluded {
+		needle := strings.ToLower(strings.TrimSpace(item))
+		if needle != "" && (base == needle || (!strings.Contains(needle, ".") && strings.Contains(base, needle))) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/internal/config/config_test.go b/internal/config/config_test.go
new file mode 100644
index 0000000..99e9574
--- /dev/null
+++ b/internal/config/config_test.go
@@ -0,0 +1,45 @@
+package config
+
+import "testing"
+
+func TestDefaultValid(t *testing.T) {
+	if err := Default().Validate(); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestLayoutFallback(t *testing.T) {
+	c := Default()
+	if _, ok := c.LayoutFor(`\\.\DISPLAY99`); !ok {
+		t.Fatal("expected wildcard layout")
+	}
+}
+
+func TestExcluded(t *testing.T) {
+	c := Default()
+	c.Excluded = []string{"notepad", "exact.exe"}
+	if !c.IsExcluded(`C:\Windows\notepad.exe`) || !c.IsExcluded(`C:\x\exact.exe`) || c.IsExcluded(`C:\x\other.exe`) {
+		t.Fatal("bad exclusion matching")
+	}
+}
+
+func TestInvalidNumberedAction(t *testing.T) {
+	c := Default()
+	c.Hotkeys = []Hotkey{{Keys: "win+alt+1", Action: "zone_nope"}}
+	if err := c.Validate(); err == nil {
+		t.Fatal("expected invalid numbered action to fail")
+	}
+}
+
+func TestOverlayValidation(t *testing.T) {
+	c := Default()
+	c.Overlay.Color = "blue"
+	if err := c.Validate(); err == nil {
+		t.Fatal("expected invalid overlay color to fail")
+	}
+	c = Default()
+	c.Overlay.Opacity = 101
+	if err := c.Validate(); err == nil {
+		t.Fatal("expected invalid overlay opacity to fail")
+	}
+}
diff --git a/internal/layout/layout.go b/internal/layout/layout.go
new file mode 100644
index 0000000..4a13aa5
--- /dev/null
+++ b/internal/layout/layout.go
@@ -0,0 +1,58 @@
+package layout
+
+import (
+	"math"
+
+	"github.com/stevec/fancywin/internal/config"
+)
+
+type Rect struct{ Left, Top, Right, Bottom int32 }
+type Point struct{ X, Y int32 }
+
+func Resolve(work Rect, z config.Zone, gap int) Rect {
+	w, h := float64(work.Right-work.Left), float64(work.Bottom-work.Top)
+	r := Rect{
+		Left:   work.Left + int32(math.Round(w*z.X/100)),
+		Top:    work.Top + int32(math.Round(h*z.Y/100)),
+		Right:  work.Left + int32(math.Round(w*(z.X+z.Width)/100)),
+		Bottom: work.Top + int32(math.Round(h*(z.Y+z.Height)/100)),
+	}
+	g := int32(gap)
+	r.Left += g
+	r.Top += g
+	r.Right -= g
+	r.Bottom -= g
+	if r.Right <= r.Left {
+		r.Right = r.Left + 1
+	}
+	if r.Bottom <= r.Top {
+		r.Bottom = r.Top + 1
+	}
+	return r
+}
+
+func Contains(r Rect, p Point) bool {
+	return p.X >= r.Left && p.X < r.Right && p.Y >= r.Top && p.Y < r.Bottom
+}
+
+func ZoneAt(work Rect, zones []config.Zone, gap int, p Point) int {
+	for i, z := range zones {
+		if Contains(Resolve(work, z, gap), p) {
+			return i
+		}
+	}
+	return -1
+}
+
+func Closest(work Rect, zones []config.Zone, p Point) int {
+	best, bestD := -1, math.MaxFloat64
+	for i, z := range zones {
+		r := Resolve(work, z, 0)
+		cx, cy := float64(r.Left+r.Right)/2, float64(r.Top+r.Bottom)/2
+		dx, dy := float64(p.X)-cx, float64(p.Y)-cy
+		if d := dx*dx + dy*dy; d < bestD {
+			best, bestD = i, d
+		}
+	}
+	return best
+}
diff --git a/internal/layout/layout_test.go b/internal/layout/layout_test.go
new file mode 100644
index 0000000..76c2e5f
--- /dev/null
+++ b/internal/layout/layout_test.go
@@ -0,0 +1,21 @@
+package layout
+
+import (
+	"github.com/stevec/fancywin/internal/config"
+	"testing"
+)
+
+func TestResolvePercentageAndGap(t *testing.T) {
+	got := Resolve(Rect{0, 0, 1920, 1040}, config.Zone{X: 50, Y: 0, Width: 50, Height: 100}, 8)
+	want := (Rect{968, 8, 1912, 1032})
+	if got != want {
+		t.Fatalf("got %+v, want %+v", got, want)
+	}
+}
+
+func TestZoneAt(t *testing.T) {
+	z := []config.Zone{{X: 0, Y: 0, Width: 50, Height: 100}, {X: 50, Y: 0, Width: 50, Height: 100}}
+	if got := ZoneAt(Rect{0, 0, 100, 100}, z, 0, Point{75, 20}); got != 1 {
+		t.Fatalf("got %d", got)
+	}
+}
diff --git a/internal/platform/overlay_windows.go b/internal/platform/overlay_windows.go
new file mode 100644
index 0000000..24a6ab2
--- /dev/null
+++ b/internal/platform/overlay_windows.go
@@ -0,0 +1,300 @@
+//go:build windows
+
+package platform
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+	"sync"
+	"unsafe"
+
+	"github.com/stevec/fancywin/internal/config"
+	"github.com/stevec/fancywin/internal/layout"
+	"golang.org/x/sys/windows"
+)
+
+const (
+	wmPaint           = 0x000F
+	wmEraseBackground = 0x0014
+	wmNcHitTest       = 0x0084
+	wsPopup           = 0x80000000
+	wsExTopmost       = 0x00000008
+	wsExTransparent   = 0x00000020
+	wsExToolWindow    = 0x00000080
+	wsExLayered       = 0x00080000
+	wsExNoActivate    = 0x08000000
+	lwaColorKey       = 0x00000001
+	lwaAlpha          = 0x00000002
+	hwndTopmost       = ^uintptr(0)
+	swpShowWindow     = 0x0040
+	psSolid           = 0
+	nullBrush         = 5
+	bkModeTransparent = 1
+	dtCenter          = 0x00000001
+	dtVCenter         = 0x00000004
+	dtSingleLine      = 0x00000020
+	dtEndEllipsis     = 0x00008000
+	fontWeightBold    = 700
+	nonAntialiased    = 3
+	overlayColorKey   = 0x00010001 // RGB(1,0,1), made fully transparent.
+)
+
+var (
+	gdi32                          = windows.NewLazySystemDLL("gdi32.dll")
+	procRegisterClassEx            = user32.NewProc("RegisterClassExW")
+	procCreateWindowEx             = user32.NewProc("CreateWindowExW")
+	procDestroyWindow              = user32.NewProc("DestroyWindow")
+	procDefWindowProc              = user32.NewProc("DefWindowProcW")
+	procBeginPaint                 = user32.NewProc("BeginPaint")
+	procEndPaint                   = user32.NewProc("EndPaint")
+	procGetClientRect              = user32.NewProc("GetClientRect")
+	procFillRect                   = user32.NewProc("FillRect")
+	procDrawText                   = user32.NewProc("DrawTextW")
+	procInvalidateRect             = user32.NewProc("InvalidateRect")
+	procUpdateWindow               = user32.NewProc("UpdateWindow")
+	procSetLayeredWindowAttributes = user32.NewProc("SetLayeredWindowAttributes")
+	procGetModuleHandle            = kernel32.NewProc("GetModuleHandleW")
+	procCreateSolidBrush           = gdi32.NewProc("CreateSolidBrush")
+	procCreatePen                  = gdi32.NewProc("CreatePen")
+	procSelectObject               = gdi32.NewProc("SelectObject")
+	procDeleteObject               = gdi32.NewProc("DeleteObject")
+	procRectangle                  = gdi32.NewProc("Rectangle")
+	procSetBkMode                  = gdi32.NewProc("SetBkMode")
+	procSetTextColor               = gdi32.NewProc("SetTextColor")
+	procCreateFont                 = gdi32.NewProc("CreateFontW")
+	procGetStockObject             = gdi32.NewProc("GetStockObject")
+	overlayClassName, _            = windows.UTF16PtrFromString("FancyWinZoneOverlay")
+	overlayWindowProc              = windows.NewCallback(overlayWndProc)
+	overlayViews                   = struct {
+		sync.RWMutex
+		items map[windows.Handle]*overlayView
+	}{items: make(map[windows.Handle]*overlayView)}
+)
+
+type windowClassEx struct {
+	Size       uint32
+	Style      uint32
+	WndProc    uintptr
+	ClsExtra   int32
+	WndExtra   int32
+	Instance   windows.Handle
+	Icon       windows.Handle
+	Cursor     windows.Handle
+	Background windows.Handle
+	MenuName   *uint16
+	ClassName  *uint16
+	IconSmall  windows.Handle
+}
+
+type paintStruct struct {
+	DC        windows.Handle
+	Erase     int32
+	Paint     rect
+	Restore   int32
+	IncUpdate int32
+	Reserved  [32]byte
+}
+
+type overlayView struct {
+	hwnd  windows.Handle
+	work  layout.Rect
+	zones []config.Zone
+	gap   int
+	style config.Overlay
+}
+
+type overlayManager struct {
+	classReady bool
+	hwnd       windows.Handle
+	monitor    windows.Handle
+	signature  string
+}
+
+func (m *overlayManager) show(mon windows.Handle, work layout.Rect, zones []config.Zone, gap int, style config.Overlay) error {
+	signature := overlaySignature(mon, work, zones, gap, style)
+	if m.hwnd != 0 && m.signature == signature {
+		return nil
+	}
+	m.hide()
+	if err := m.ensureClass(); err != nil {
+		return err
+	}
+
+	instance, _, instanceErr := procGetModuleHandle.Call(0)
+	if instance == 0 {
+		return fmt.Errorf("GetModuleHandleW: %v", instanceErr)
+	}
+	w, h := work.Right-work.Left, work.Bottom-work.Top
+	raw, _, createErr := procCreateWindowEx.Call(
+		wsExTopmost|wsExTransparent|wsExToolWindow|wsExLayered|wsExNoActivate,
+		uintptr(unsafe.Pointer(overlayClassName)),
+		uintptr(unsafe.Pointer(overlayClassName)),
+		wsPopup,
+		uintptr(work.Left), uintptr(work.Top), uintptr(w), uintptr(h),
+		0, 0, instance, 0,
+	)
+	if raw == 0 {
+		return fmt.Errorf("CreateWindowExW: %v", createErr)
+	}
+	hwnd := windows.Handle(raw)
+	view := &overlayView{hwnd: hwnd, work: work, zones: append([]config.Zone(nil), zones...), gap: gap, style: style}
+	overlayViews.Lock()
+	overlayViews.items[hwnd] = view
+	overlayViews.Unlock()
+
+	alpha := byte((style.Opacity*255 + 50) / 100)
+	if ok, _, err := procSetLayeredWindowAttributes.Call(raw, overlayColorKey, uintptr(alpha), lwaColorKey|lwaAlpha); ok == 0 {
+		m.destroy(hwnd)
+		return fmt.Errorf("SetLayeredWindowAttributes: %v", err)
+	}
+	procInvalidateRect.Call(raw, 0, 1)
+	procUpdateWindow.Call(raw)
+	if ok, _, err := procSetWindowPos.Call(raw, hwndTopmost, uintptr(work.Left), uintptr(work.Top), uintptr(w), uintptr(h), swpNoActivate|swpNoOwnerZOrder|swpShowWindow); ok == 0 {
+		m.destroy(hwnd)
+		return fmt.Errorf("show overlay: %v", err)
+	}
+	m.hwnd = hwnd
+	m.monitor = mon
+	m.signature = signature
+	return nil
+}
+
+func (m *overlayManager) hide() {
+	if m.hwnd != 0 {
+		m.destroy(m.hwnd)
+	}
+	m.hwnd = 0
+	m.monitor = 0
+	m.signature = ""
+}
+
+func (m *overlayManager) destroy(hwnd windows.Handle) {
+	overlayViews.Lock()
+	delete(overlayViews.items, hwnd)
+	overlayViews.Unlock()
+	procDestroyWindow.Call(uintptr(hwnd))
+}
+
+func (m *overlayManager) ensureClass() error {
+	if m.classReady {
+		return nil
+	}
+	instance, _, instanceErr := procGetModuleHandle.Call(0)
+	if instance == 0 {
+		return fmt.Errorf("GetModuleHandleW: %v", instanceErr)
+	}
+	wc := windowClassEx{
+		Size:      uint32(unsafe.Sizeof(windowClassEx{})),
+		WndProc:   overlayWindowProc,
+		Instance:  windows.Handle(instance),
+		ClassName: overlayClassName,
+	}
+	if atom, _, err := procRegisterClassEx.Call(uintptr(unsafe.Pointer(&wc))); atom == 0 {
+		return fmt.Errorf("RegisterClassExW: %v", err)
+	}
+	m.classReady = true
+	return nil
+}
+
+func overlayWndProc(rawHWND, message, wParam, lParam uintptr) uintptr {
+	hwnd := windows.Handle(rawHWND)
+	switch message {
+	case wmNcHitTest:
+		return ^uintptr(0) // HTTRANSPARENT: always pass pointer input through.
+	case wmEraseBackground:
+		return 1
+	case wmPaint:
+		overlayViews.RLock()
+		view := overlayViews.items[hwnd]
+		overlayViews.RUnlock()
+		if view != nil {
+			view.paint()
+			return 0
+		}
+	}
+	r, _, _ := procDefWindowProc.Call(rawHWND, message, wParam, lParam)
+	return r
+}
+
+func (v *overlayView) paint() {
+	var ps paintStruct
+	dc, _, _ := procBeginPaint.Call(uintptr(v.hwnd), uintptr(unsafe.Pointer(&ps)))
+	if dc == 0 {
+		return
+	}
+	defer procEndPaint.Call(uintptr(v.hwnd), uintptr(unsafe.Pointer(&ps)))
+
+	var client rect
+	procGetClientRect.Call(uintptr(v.hwnd), uintptr(unsafe.Pointer(&client)))
+	background, _, _ := procCreateSolidBrush.Call(overlayColorKey)
+	if background != 0 {
+		procFillRect.Call(dc, uintptr(unsafe.Pointer(&client)), background)
+		procDeleteObject.Call(background)
+	}
+
+	color := colorRef(v.style.Color)
+	pen, _, _ := procCreatePen.Call(psSolid, uintptr(v.style.BorderWidth), uintptr(color))
+	nullFill, _, _ := procGetStockObject.Call(nullBrush)
+	oldPen, _, _ := procSelectObject.Call(dc, pen)
+	oldBrush, _, _ := procSelectObject.Call(dc, nullFill)
+	defer func() {
+		procSelectObject.Call(dc, oldPen)
+		procSelectObject.Call(dc, oldBrush)
+		if pen != 0 {
+			procDeleteObject.Call(pen)
+		}
+	}()
+
+	fontName, _ := windows.UTF16PtrFromString("Segoe UI")
+	font, _, _ := procCreateFont.Call(^uintptr(55), 0, 0, 0, fontWeightBold, 0, 0, 0, 1, 0, 0, nonAntialiased, 0, uintptr(unsafe.Pointer(fontName)))
+	oldFont := uintptr(0)
+	if font != 0 {
+		oldFont, _, _ = procSelectObject.Call(dc, font)
+		defer func() { procSelectObject.Call(dc, oldFont); procDeleteObject.Call(font) }()
+	}
+	procSetBkMode.Call(dc, bkModeTransparent)
+
+	for i, zone := range v.zones {
+		r := layout.Resolve(v.work, zone, v.gap)
+		r.Left -= v.work.Left
+		r.Right -= v.work.Left
+		r.Top -= v.work.Top
+		r.Bottom -= v.work.Top
+		procRectangle.Call(dc, uintptr(r.Left), uintptr(r.Top), uintptr(r.Right), uintptr(r.Bottom))
+		name := strings.TrimSpace(zone.Name)
+		if name == "" {
+			name = fmt.Sprintf("Zone %d", i+1)
+		}
+		text, err := windows.UTF16FromString(name)
+		if err != nil || len(text) == 0 {
+			continue
+		}
+		shadow := rect{Left: r.Left + 2, Top: r.Top + 2, Right: r.Right + 2, Bottom: r.Bottom + 2}
+		procSetTextColor.Call(dc, 0x00000000)
+		procDrawText.Call(dc, uintptr(unsafe.Pointer(&text[0])), ^uintptr(0), uintptr(unsafe.Pointer(&shadow)), dtCenter|dtVCenter|dtSingleLine|dtEndEllipsis)
+		label := rect{Left: r.Left, Top: r.Top, Right: r.Right, Bottom: r.Bottom}
+		procSetTextColor.Call(dc, uintptr(color))
+		procDrawText.Call(dc, uintptr(unsafe.Pointer(&text[0])), ^uintptr(0), uintptr(unsafe.Pointer(&label)), dtCenter|dtVCenter|dtSingleLine|dtEndEllipsis)
+	}
+}
+
+func colorRef(value string) uint32 {
+	rgb, err := strconv.ParseUint(strings.TrimPrefix(value, "#"), 16, 24)
+	if err != nil {
+		return 0x00EFAE00
+	}
+	r := uint32(rgb >> 16)
+	g := uint32((rgb >> 8) & 0xff)
+	b := uint32(rgb & 0xff)
+	return r | g<<8 | b<<16
+}
+
+func overlaySignature(mon windows.Handle, work layout.Rect, zones []config.Zone, gap int, style config.Overlay) string {
+	var b strings.Builder
+	fmt.Fprintf(&b, "%x|%v|%d|%v", uintptr(mon), work, gap, style)
+	for _, zone := range zones {
+		fmt.Fprintf(&b, "|%s:%g:%g:%g:%g", zone.Name, zone.X, zone.Y, zone.Width, zone.Height)
+	}
+	return b.String()
+}
diff --git a/internal/platform/platform_other.go b/internal/platform/platform_other.go
new file mode 100644
index 0000000..4facc46
--- /dev/null
+++ b/internal/platform/platform_other.go
@@ -0,0 +1,12 @@
+//go:build !windows
+
+package platform
+
+import (
+	"errors"
+	"github.com/stevec/fancywin/internal/config"
+)
+
+func Run(_ string, _ config.Config, _ bool) error {
+	return errors.New("the window manager can only run on Windows")
+}
diff --git a/internal/platform/platform_windows.go b/internal/platform/platform_windows.go
new file mode 100644
index 0000000..fd8678c
--- /dev/null
+++ b/internal/platform/platform_windows.go
@@ -0,0 +1,694 @@
+//go:build windows
+
+package platform
+
+import (
+	"fmt"
+	"log"
+	"os"
+	"runtime"
+	"strconv"
+	"strings"
+	"sync"
+	"time"
+	"unsafe"
+
+	"github.com/stevec/fancywin/internal/config"
+	"github.com/stevec/fancywin/internal/layout"
+	"golang.org/x/sys/windows"
+)
+
+const (
+	eventSystemMoveSizeStart = 0x000A
+	eventSystemMoveSizeEnd   = 0x000B
+	wineventOutOfContext     = 0x0000
+	wineventSkipOwnProcess   = 0x0002
+	objectIDWindow           = 0
+	wmHotkey                 = 0x0312
+	wmTimer                  = 0x0113
+	wmQuit                   = 0x0012
+	monitorDefaultToNearest  = 2
+	monitorDefaultToNull     = 0
+	modAlt                   = 0x0001
+	modControl               = 0x0002
+	modShift                 = 0x0004
+	modWin                   = 0x0008
+	modNoRepeat              = 0x4000
+	vkLButton                = 0x01
+	vkShift                  = 0x10
+	gaRoot                   = 2
+	gwOwner                  = 4
+	wsChild                  = 0x40000000
+	swRestore                = 9
+	swpNoZOrder              = 0x0004
+	swpNoActivate            = 0x0010
+	swpNoOwnerZOrder         = 0x0200
+)
+
+var (
+	user32                       = windows.NewLazySystemDLL("user32.dll")
+	kernel32                     = windows.NewLazySystemDLL("kernel32.dll")
+	dwmapi                       = windows.NewLazySystemDLL("dwmapi.dll")
+	procSetWinEventHook          = user32.NewProc("SetWinEventHook")
+	procUnhookWinEvent           = user32.NewProc("UnhookWinEvent")
+	procGetMessage               = user32.NewProc("GetMessageW")
+	procTranslateMessage         = user32.NewProc("TranslateMessage")
+	procDispatchMessage          = user32.NewProc("DispatchMessageW")
+	procRegisterHotKey           = user32.NewProc("RegisterHotKey")
+	procUnregisterHotKey         = user32.NewProc("UnregisterHotKey")
+	procGetForegroundWindow      = user32.NewProc("GetForegroundWindow")
+	procGetCursorPos             = user32.NewProc("GetCursorPos")
+	procWindowFromPoint          = user32.NewProc("WindowFromPoint")
+	procMonitorFromPoint         = user32.NewProc("MonitorFromPoint")
+	procMonitorFromWindow        = user32.NewProc("MonitorFromWindow")
+	procGetMonitorInfo           = user32.NewProc("GetMonitorInfoW")
+	procGetAsyncKeyState         = user32.NewProc("GetAsyncKeyState")
+	procSetWindowPos             = user32.NewProc("SetWindowPos")
+	procShowWindow               = user32.NewProc("ShowWindow")
+	procIsWindowVisible          = user32.NewProc("IsWindowVisible")
+	procGetAncestor              = user32.NewProc("GetAncestor")
+	procGetWindow                = user32.NewProc("GetWindow")
+	procGetWindowLongPtr         = user32.NewProc("GetWindowLongPtrW")
+	procGetWindowRect            = user32.NewProc("GetWindowRect")
+	procGetWindowThreadProcessID = user32.NewProc("GetWindowThreadProcessId")
+	procSetTimer                 = user32.NewProc("SetTimer")
+	procKillTimer                = user32.NewProc("KillTimer")
+	procSetProcessDPIAwareV2     = user32.NewProc("SetProcessDpiAwarenessContext")
+	procEnumDisplayMonitors      = user32.NewProc("EnumDisplayMonitors")
+	procCreateMutex              = kernel32.NewProc("CreateMutexW")
+	procDwmGetWindowAttribute    = dwmapi.NewProc("DwmGetWindowAttribute")
+)
+
+type point struct{ X, Y int32 }
+type rect struct{ Left, Top, Right, Bottom int32 }
+type message struct {
+	HWnd    windows.Handle
+	Message uint32
+	_       uint32
+	WParam  uintptr
+	LParam  uintptr
+	Time    uint32
+	Pt      point
+	Private uint32
+}
+type monitorInfoEx struct {
+	Size    uint32
+	Monitor rect
+	Work    rect
+	Flags   uint32
+	Device  [32]uint16
+}
+type hotkeyBinding struct {
+	id     int
+	action string
+	keys   string
+}
+type engine struct {
+	mu              sync.RWMutex
+	configPath      string
+	cfg             config.Config
+	configTime      time.Time
+	dragging        windows.Handle
+	dragShift       bool
+	mouseDown       bool
+	mouseWindow     windows.Handle
+	mouseStart      rect
+	mouseMoved      bool
+	mouseShift      bool
+	fallbackHandled windows.Handle
+	hook            windows.Handle
+	hotkeys         map[int]hotkeyBinding
+	debug           bool
+	overlay         *overlayManager
+	overlayFailed   bool
+}
+
+var activeEngine *engine
+
+func Run(configPath string, cfg config.Config, debug bool) error {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+	procSetProcessDPIAwareV2.Call(^uintptr(3)) // DPI_AWARENESS_CONTEXT_PER_MONITOR_AWARE_V2 (-4)
+
+	name, _ := windows.UTF16PtrFromString("Local\\FancyWin-Portable-Window-Manager")
+	mutex, _, createErr := procCreateMutex.Call(0, 0, uintptr(unsafe.Pointer(name)))
+	if mutex == 0 {
+		return fmt.Errorf("create single-instance mutex: %v", createErr)
+	}
+	defer windows.CloseHandle(windows.Handle(mutex))
+	if createErr == windows.ERROR_ALREADY_EXISTS {
+		return fmt.Errorf("another instance is already running")
+	}
+
+	e := &engine{configPath: configPath, cfg: cfg, hotkeys: make(map[int]hotkeyBinding), debug: debug, overlay: &overlayManager{}}
+	defer e.overlay.hide()
+	e.rememberConfigTime()
+	activeEngine = e
+	defer func() { activeEngine = nil }()
+
+	callback := windows.NewCallback(winEventCallback)
+	hook, _, callErr := procSetWinEventHook.Call(eventSystemMoveSizeStart, eventSystemMoveSizeEnd, 0, callback, 0, 0, wineventOutOfContext|wineventSkipOwnProcess)
+	if hook == 0 {
+		return fmt.Errorf("install window move hook: %v", callErr)
+	}
+	e.hook = windows.Handle(hook)
+	defer procUnhookWinEvent.Call(hook)
+	if err := e.registerHotkeys(); err != nil {
+		return err
+	}
+	defer e.unregisterHotkeys()
+	configTimer, _, configTimerErr := procSetTimer.Call(0, 1, 2000, 0)
+	if configTimer == 0 {
+		return fmt.Errorf("create configuration timer: %v", configTimerErr)
+	}
+	defer procKillTimer.Call(0, configTimer)
+	shiftTimer, _, shiftTimerErr := procSetTimer.Call(0, 2, 25, 0)
+	if shiftTimer == 0 {
+		return fmt.Errorf("create drag activation timer: %v", shiftTimerErr)
+	}
+	defer procKillTimer.Call(0, shiftTimer)
+
+	log.Printf("ready: Shift+drag snapping=%v, overlay=%v, %d hotkeys", cfg.ShiftDrag, cfg.Overlay.Enabled, len(e.hotkeys))
+	logMonitors()
+	var msg message
+	for {
+		r, _, err := procGetMessage.Call(uintptr(unsafe.Pointer(&msg)), 0, 0, 0)
+		if int32(r) == -1 {
+			return fmt.Errorf("message loop: %v", err)
+		}
+		if r == 0 || msg.Message == wmQuit {
+			return nil
+		}
+		switch msg.Message {
+		case wmHotkey:
+			e.handleHotkey(int(msg.WParam))
+		case wmTimer:
+			switch msg.WParam {
+			case configTimer:
+				e.reloadIfChanged()
+			case shiftTimer:
+				e.pollInput()
+			}
+		}
+		procTranslateMessage.Call(uintptr(unsafe.Pointer(&msg)))
+		procDispatchMessage.Call(uintptr(unsafe.Pointer(&msg)))
+	}
+}
+
+func winEventCallback(_, event, rawHWND, idObject, idChild, _, _ uintptr) uintptr {
+	e := activeEngine
+	hwnd := windows.Handle(rawHWND)
+	if e == nil || hwnd == 0 || idObject != objectIDWindow || idChild != 0 {
+		return 0
+	}
+	switch event {
+	case eventSystemMoveSizeStart:
+		if e.canManage(hwnd) {
+			e.fallbackHandled = 0
+			e.dragging = hwnd
+			e.dragShift = keyDown(vkShift)
+			e.debugf("move started: hwnd=0x%x app=%q shift=%v", uintptr(hwnd), executableForWindow(hwnd), e.dragShift)
+		} else {
+			e.debugf("move ignored: hwnd=0x%x app=%q is not a manageable top-level window", uintptr(hwnd), executableForWindow(hwnd))
+		}
+	case eventSystemMoveSizeEnd:
+		if e.dragging == hwnd {
+			if e.fallbackHandled == hwnd {
+				e.dragging = 0
+				e.dragShift = false
+				e.fallbackHandled = 0
+				e.debugf("native move end arrived after mouse fallback: hwnd=0x%x", uintptr(hwnd))
+				break
+			}
+			activated := e.dragShift || keyDown(vkShift)
+			e.dragging = 0
+			e.dragShift = false
+			if e.mouseWindow == hwnd {
+				e.mouseWindow = 0 // The native event path owns this drag.
+				e.mouseMoved = false
+			}
+			e.mu.RLock()
+			enabled := e.cfg.ShiftDrag
+			e.mu.RUnlock()
+			if enabled && activated {
+				if ok, reason := e.snapAtCursor(hwnd); !ok {
+					log.Printf("window was not snapped: %s", reason)
+				} else {
+					e.debugf("window snapped: hwnd=0x%x", uintptr(hwnd))
+				}
+			} else {
+				e.debugf("move ended without snapping: hwnd=0x%x shift_drag=%v shift=%v", uintptr(hwnd), enabled, activated)
+			}
+		} else {
+			e.debugf("move end ignored: hwnd=0x%x did not match active drag 0x%x", uintptr(hwnd), uintptr(e.dragging))
+		}
+	}
+	return 0
+}
+
+func (e *engine) pollInput() {
+	if e.dragging != 0 {
+		e.dragShift = keyDown(vkShift)
+	}
+
+	down := keyDown(vkLButton)
+	switch {
+	case down && !e.mouseDown:
+		e.beginMouseTracking()
+	case down && e.mouseDown:
+		e.updateMouseTracking()
+	case !down && e.mouseDown:
+		e.endMouseTracking()
+	}
+	e.mouseDown = down
+	e.updateOverlay()
+}
+
+func (e *engine) updateOverlay() {
+	hide := func() {
+		if e.overlay.hwnd != 0 {
+			e.debugf("zone overlay hidden")
+		}
+		e.overlay.hide()
+	}
+	e.mu.RLock()
+	cfg := e.cfg
+	e.mu.RUnlock()
+	activeDrag := e.mouseDown && (e.dragging != 0 || (e.mouseWindow != 0 && e.mouseMoved))
+	if !cfg.Overlay.Enabled || !activeDrag || !keyDown(vkShift) {
+		hide()
+		return
+	}
+	var p point
+	if ok, _, _ := procGetCursorPos.Call(uintptr(unsafe.Pointer(&p))); ok == 0 {
+		hide()
+		return
+	}
+	mon, _, _ := procMonitorFromPoint.Call(packPoint(p), monitorDefaultToNull)
+	if mon == 0 {
+		hide()
+		return
+	}
+	mi, ok := monitorDetails(windows.Handle(mon))
+	if !ok {
+		hide()
+		return
+	}
+	selected, ok := cfg.LayoutFor(mi.device())
+	if !ok {
+		hide()
+		return
+	}
+	wasVisible := e.overlay.hwnd != 0
+	if err := e.overlay.show(windows.Handle(mon), toLayoutRect(mi.Work), selected.Zones, cfg.Gap, cfg.Overlay); err != nil {
+		hide()
+		if !e.overlayFailed {
+			log.Printf("cannot show zone overlay: %v", err)
+			e.overlayFailed = true
+		}
+	} else if !wasVisible {
+		e.debugf("zone overlay shown: monitor=%s zones=%d", mi.device(), len(selected.Zones))
+	}
+}
+
+func (e *engine) beginMouseTracking() {
+	var p point
+	if ok, _, _ := procGetCursorPos.Call(uintptr(unsafe.Pointer(&p))); ok == 0 {
+		return
+	}
+	raw, _, _ := procWindowFromPoint.Call(packPoint(p))
+	if raw == 0 {
+		e.debugf("mouse fallback: no window under press at (%d,%d)", p.X, p.Y)
+		return
+	}
+	root, _, _ := procGetAncestor.Call(raw, gaRoot)
+	if root == 0 {
+		root = raw
+	}
+	hwnd := windows.Handle(root)
+	if !e.canManage(hwnd) {
+		e.debugf("mouse fallback: window under press is not manageable: hwnd=0x%x app=%q", uintptr(hwnd), executableForWindow(hwnd))
+		return
+	}
+	var start rect
+	if ok, _, _ := procGetWindowRect.Call(uintptr(hwnd), uintptr(unsafe.Pointer(&start))); ok == 0 {
+		e.debugf("mouse fallback: GetWindowRect failed for hwnd=0x%x", uintptr(hwnd))
+		return
+	}
+	e.mouseWindow = hwnd
+	e.mouseStart = start
+	e.mouseMoved = false
+	e.mouseShift = keyDown(vkShift)
+	e.debugf("mouse fallback tracking started: hwnd=0x%x app=%q shift=%v", uintptr(hwnd), executableForWindow(hwnd), e.mouseShift)
+}
+
+func (e *engine) updateMouseTracking() {
+	if e.mouseWindow == 0 {
+		return
+	}
+	e.mouseShift = keyDown(vkShift)
+	var current rect
+	if ok, _, _ := procGetWindowRect.Call(uintptr(e.mouseWindow), uintptr(unsafe.Pointer(&current))); ok != 0 && rectMoved(e.mouseStart, current) {
+		e.mouseMoved = true
+	}
+}
+
+func (e *engine) endMouseTracking() {
+	hwnd := e.mouseWindow
+	moved := e.mouseMoved
+	activated := e.mouseShift || keyDown(vkShift)
+	e.mouseWindow = 0
+	e.mouseMoved = false
+	e.mouseShift = false
+	if hwnd == 0 {
+		return
+	}
+	if !moved {
+		e.debugf("mouse fallback ignored click: hwnd=0x%x rectangle did not move", uintptr(hwnd))
+		return
+	}
+	e.mu.RLock()
+	enabled := e.cfg.ShiftDrag
+	e.mu.RUnlock()
+	if !enabled || !activated {
+		e.debugf("mouse fallback move ended without snapping: hwnd=0x%x shift_drag=%v shift=%v", uintptr(hwnd), enabled, activated)
+		return
+	}
+	if e.dragging == hwnd {
+		e.fallbackHandled = hwnd
+	}
+	if ok, reason := e.snapAtCursor(hwnd); !ok {
+		log.Printf("window was not snapped by mouse fallback: %s", reason)
+	} else {
+		e.debugf("window snapped by mouse fallback: hwnd=0x%x", uintptr(hwnd))
+	}
+}
+
+func rectMoved(a, b rect) bool {
+	const threshold = int32(2)
+	return abs32(a.Left-b.Left) > threshold || abs32(a.Top-b.Top) > threshold ||
+		abs32(a.Right-b.Right) > threshold || abs32(a.Bottom-b.Bottom) > threshold
+}
+
+func abs32(v int32) int32 {
+	if v < 0 {
+		return -v
+	}
+	return v
+}
+
+func packPoint(p point) uintptr {
+	return uintptr(uint32(p.X)) | uintptr(uint64(uint32(p.Y))<<32)
+}
+
+func (e *engine) snapAtCursor(hwnd windows.Handle) (bool, string) {
+	var p point
+	if ok, _, _ := procGetCursorPos.Call(uintptr(unsafe.Pointer(&p))); ok == 0 {
+		return false, "GetCursorPos failed"
+	}
+	mon, _, _ := procMonitorFromPoint.Call(packPoint(p), monitorDefaultToNull)
+	if mon == 0 {
+		return false, fmt.Sprintf("no monitor contains pointer (%d,%d)", p.X, p.Y)
+	}
+	mi, ok := monitorDetails(windows.Handle(mon))
+	if !ok {
+		return false, "GetMonitorInfo failed"
+	}
+	e.mu.RLock()
+	cfg := e.cfg
+	e.mu.RUnlock()
+	l, ok := cfg.LayoutFor(mi.device())
+	if !ok {
+		return false, fmt.Sprintf("no layout matches monitor %s and no wildcard layout exists", mi.device())
+	}
+	work := toLayoutRect(mi.Work)
+	zi := layout.ZoneAt(work, l.Zones, 0, layout.Point{X: p.X, Y: p.Y})
+	if zi < 0 {
+		return false, fmt.Sprintf("pointer (%d,%d) is outside every zone on %s", p.X, p.Y, mi.device())
+	}
+	e.debugf("snap target: monitor=%s zone=%d name=%q pointer=(%d,%d)", mi.device(), zi+1, l.Zones[zi].Name, p.X, p.Y)
+	if !e.moveTo(hwnd, work, l.Zones[zi], cfg.Gap) {
+		return false, "SetWindowPos failed (the window may be elevated or may reject resizing)"
+	}
+	return true, ""
+}
+
+func (e *engine) handleHotkey(id int) {
+	binding, ok := e.hotkeys[id]
+	if !ok {
+		return
+	}
+	h, _, _ := procGetForegroundWindow.Call()
+	hwnd := windows.Handle(h)
+	if !e.canManage(hwnd) {
+		return
+	}
+	mon, _, _ := procMonitorFromWindow.Call(uintptr(hwnd), monitorDefaultToNearest)
+	if mon == 0 {
+		return
+	}
+	mi, ok := monitorDetails(windows.Handle(mon))
+	if !ok {
+		return
+	}
+	e.mu.RLock()
+	cfg := e.cfg
+	e.mu.RUnlock()
+	l, ok := cfg.LayoutFor(mi.device())
+	if !ok || len(l.Zones) == 0 {
+		return
+	}
+	idx := -1
+	action := strings.ToLower(strings.TrimSpace(binding.action))
+	if strings.HasPrefix(action, "zone_") {
+		n, err := strconv.Atoi(strings.TrimPrefix(action, "zone_"))
+		if err == nil {
+			idx = n - 1
+		}
+	} else {
+		var wr rect
+		if r, _, _ := procGetWindowRect.Call(uintptr(hwnd), uintptr(unsafe.Pointer(&wr))); r == 0 {
+			return
+		}
+		center := layout.Point{X: (wr.Left + wr.Right) / 2, Y: (wr.Top + wr.Bottom) / 2}
+		current := layout.Closest(toLayoutRect(mi.Work), l.Zones, center)
+		if action == "next_zone" {
+			idx = (current + 1) % len(l.Zones)
+		} else {
+			idx = (current - 1 + len(l.Zones)) % len(l.Zones)
+		}
+	}
+	if idx >= 0 && idx < len(l.Zones) {
+		e.moveTo(hwnd, toLayoutRect(mi.Work), l.Zones[idx], cfg.Gap)
+	}
+}
+
+func (e *engine) moveTo(hwnd windows.Handle, work layout.Rect, z config.Zone, gap int) bool {
+	r := layout.Resolve(work, z, gap)
+	procShowWindow.Call(uintptr(hwnd), swRestore)
+	r = outerRectForVisibleFrame(hwnd, r)
+	ok, _, err := procSetWindowPos.Call(uintptr(hwnd), 0, uintptr(r.Left), uintptr(r.Top), uintptr(r.Right-r.Left), uintptr(r.Bottom-r.Top), swpNoZOrder|swpNoActivate|swpNoOwnerZOrder)
+	if ok == 0 {
+		log.Printf("cannot move window 0x%x: %v (it may be elevated)", uintptr(hwnd), err)
+		return false
+	}
+	return true
+}
+
+func (e *engine) debugf(format string, args ...any) {
+	if e.debug {
+		log.Printf("debug: "+format, args...)
+	}
+}
+
+// Windows 11 windows have an invisible resize border outside the DWM-rendered
+// frame. Expand the SetWindowPos rectangle so the visible frame, rather than
+// that hidden border, aligns with the configured zone.
+func outerRectForVisibleFrame(hwnd windows.Handle, target layout.Rect) layout.Rect {
+	var outer, frame rect
+	if ok, _, _ := procGetWindowRect.Call(uintptr(hwnd), uintptr(unsafe.Pointer(&outer))); ok == 0 {
+		return target
+	}
+	const dwmwaExtendedFrameBounds = 9
+	hr, _, _ := procDwmGetWindowAttribute.Call(uintptr(hwnd), dwmwaExtendedFrameBounds, uintptr(unsafe.Pointer(&frame)), unsafe.Sizeof(frame))
+	if int32(hr) < 0 {
+		return target
+	}
+	return layout.Rect{
+		Left:   target.Left - (frame.Left - outer.Left),
+		Top:    target.Top - (frame.Top - outer.Top),
+		Right:  target.Right + (outer.Right - frame.Right),
+		Bottom: target.Bottom + (outer.Bottom - frame.Bottom),
+	}
+}
+
+func (e *engine) canManage(hwnd windows.Handle) bool {
+	if hwnd == 0 {
+		return false
+	}
+	visible, _, _ := procIsWindowVisible.Call(uintptr(hwnd))
+	if visible == 0 {
+		return false
+	}
+	root, _, _ := procGetAncestor.Call(uintptr(hwnd), gaRoot)
+	if root != uintptr(hwnd) {
+		return false
+	}
+	owner, _, _ := procGetWindow.Call(uintptr(hwnd), gwOwner)
+	if owner != 0 {
+		return false
+	}
+	style, _, _ := procGetWindowLongPtr.Call(uintptr(hwnd), ^uintptr(15))
+	if style&wsChild != 0 {
+		return false
+	}
+	exe := executableForWindow(hwnd)
+	e.mu.RLock()
+	excluded := e.cfg.IsExcluded(exe)
+	e.mu.RUnlock()
+	return !excluded
+}
+
+func executableForWindow(hwnd windows.Handle) string {
+	var pid uint32
+	procGetWindowThreadProcessID.Call(uintptr(hwnd), uintptr(unsafe.Pointer(&pid)))
+	if pid == 0 {
+		return ""
+	}
+	h, err := windows.OpenProcess(windows.PROCESS_QUERY_LIMITED_INFORMATION, false, pid)
+	if err != nil {
+		return ""
+	}
+	defer windows.CloseHandle(h)
+	buf := make([]uint16, 32768)
+	n := uint32(len(buf))
+	if err := windows.QueryFullProcessImageName(h, 0, &buf[0], &n); err != nil {
+		return ""
+	}
+	return windows.UTF16ToString(buf[:n])
+}
+
+func monitorDetails(mon windows.Handle) (monitorInfoEx, bool) {
+	mi := monitorInfoEx{Size: uint32(unsafe.Sizeof(monitorInfoEx{}))}
+	ok, _, _ := procGetMonitorInfo.Call(uintptr(mon), uintptr(unsafe.Pointer(&mi)))
+	return mi, ok != 0
+}
+func logMonitors() {
+	callback := windows.NewCallback(func(mon, _, _, _ uintptr) uintptr {
+		if mi, ok := monitorDetails(windows.Handle(mon)); ok {
+			log.Printf("monitor %s work-area=(%d,%d)-(%d,%d)", mi.device(), mi.Work.Left, mi.Work.Top, mi.Work.Right, mi.Work.Bottom)
+		}
+		return 1
+	})
+	procEnumDisplayMonitors.Call(0, 0, callback, 0)
+}
+func (m monitorInfoEx) device() string { return windows.UTF16ToString(m.Device[:]) }
+func toLayoutRect(r rect) layout.Rect {
+	return layout.Rect{Left: r.Left, Top: r.Top, Right: r.Right, Bottom: r.Bottom}
+}
+func keyDown(vk uintptr) bool { r, _, _ := procGetAsyncKeyState.Call(vk); return r&0x8000 != 0 }
+
+func (e *engine) registerHotkeys() error {
+	for i, h := range e.cfg.Hotkeys {
+		mods, vk, err := parseHotkey(h.Keys)
+		if err != nil {
+			e.unregisterHotkeys()
+			return fmt.Errorf("hotkey %q: %w", h.Keys, err)
+		}
+		id := i + 1
+		ok, _, callErr := procRegisterHotKey.Call(0, uintptr(id), uintptr(mods|modNoRepeat), uintptr(vk))
+		if ok == 0 {
+			e.unregisterHotkeys()
+			return fmt.Errorf("register hotkey %q (already in use?): %v", h.Keys, callErr)
+		}
+		e.hotkeys[id] = hotkeyBinding{id: id, action: h.Action, keys: h.Keys}
+	}
+	return nil
+}
+func (e *engine) unregisterHotkeys() {
+	for id := range e.hotkeys {
+		procUnregisterHotKey.Call(0, uintptr(id))
+		delete(e.hotkeys, id)
+	}
+}
+
+func parseHotkey(s string) (uint32, uint32, error) {
+	parts := strings.Split(strings.ToLower(strings.ReplaceAll(s, " ", "")), "+")
+	var mods, vk uint32
+	regularKeys := 0
+	for _, p := range parts {
+		switch p {
+		case "win", "windows":
+			mods |= modWin
+		case "alt":
+			mods |= modAlt
+		case "ctrl", "control":
+			mods |= modControl
+		case "shift":
+			mods |= modShift
+		case "left":
+			vk = 0x25
+			regularKeys++
+		case "up":
+			vk = 0x26
+			regularKeys++
+		case "right":
+			vk = 0x27
+			regularKeys++
+		case "down":
+			vk = 0x28
+			regularKeys++
+		case "pageup", "pgup":
+			vk = 0x21
+			regularKeys++
+		case "pagedown", "pgdn":
+			vk = 0x22
+			regularKeys++
+		default:
+			if len(p) == 1 && ((p[0] >= 'a' && p[0] <= 'z') || (p[0] >= '0' && p[0] <= '9')) {
+				vk = uint32(strings.ToUpper(p)[0])
+				regularKeys++
+			} else {
+				return 0, 0, fmt.Errorf("unknown key %q", p)
+			}
+		}
+	}
+	if mods == 0 || vk == 0 || regularKeys != 1 {
+		return 0, 0, fmt.Errorf("must contain modifiers and exactly one regular key")
+	}
+	return mods, vk, nil
+}
+
+func (e *engine) rememberConfigTime() {
+	if s, err := os.Stat(e.configPath); err == nil {
+		e.configTime = s.ModTime()
+	}
+}
+func (e *engine) reloadIfChanged() {
+	s, err := os.Stat(e.configPath)
+	if err != nil || !s.ModTime().After(e.configTime) {
+		return
+	}
+	e.configTime = s.ModTime()
+	cfg, err := config.Load(e.configPath)
+	if err != nil {
+		log.Printf("configuration reload rejected: %v", err)
+		return
+	}
+	e.unregisterHotkeys()
+	e.overlay.hide()
+	e.overlayFailed = false
+	e.mu.Lock()
+	old := e.cfg
+	e.cfg = cfg
+	e.mu.Unlock()
+	if err := e.registerHotkeys(); err != nil {
+		log.Printf("configuration hotkeys rejected: %v; restoring previous configuration", err)
+		e.mu.Lock()
+		e.cfg = old
+		e.mu.Unlock()
+		_ = e.registerHotkeys()
+		return
+	}
+	log.Printf("configuration reloaded")
+}