feat(windows): grid and recursive-grid transition animation

docs/CROSS_PLATFORM.md

@@ -923,7 +923,7 @@ important thing to know before touching overlay code:
 
 | Animation                    | macOS                                | Linux X11 / Wayland                | Windows                            |
 | ---------------------------- | ------------------------------------ | ---------------------------------- | ---------------------------------- |
-| **Grid transition**          | CoreAnimation, ease-in-out @120Hz    | goroutine, smoothstep @120fps      | ❌                                 |
+| **Grid transition**          | NSTimer @120Hz, ease-in-out, full redraw | goroutine, ease-in-out @120fps  | goroutine, ease-in-out @120fps, presented on the UI thread |
 | **Mouse action indicator**   | `CABasicAnimation` (scale + opacity) | goroutine, scale + opacity @120fps | goroutine, cubic easing @60fps     |
 | **Smooth cursor**            | ✅ stepped linear interpolation      | ✅ stepped linear interpolation    | ❌                                 |
 | **Smooth scroll**            | ✅ ease-out cubic                    | ❌                                 | ❌                                 |
@@ -946,10 +946,9 @@ discovery rather than the mode itself.
 | **Hints**         | Search input badge             | ✅                         | ✅ Cairo badge             | ✅                          |
 | **Hints**         | Label arrow / tail             | ✅ NSBezierPath            | ✅ Cairo triangle          | ✅ sampled triangle, see below |
 | **Hints**         | Label placement                | ✅ top / center / bottom   | ✅ top / center / bottom   | ✅ top / center / bottom   |
-| **Grid**          | Transition animation           | ✅                         | ✅                         | ❌                          |
 | **Grid**          | Virtual pointer indicator      | ✅                         | ✅                         | ✅                          |
 | **Grid**          | What an open subgrid shows     | ✅ the subgrid alone       | ✅ the subgrid alone       | ⚠️ the parent cells return under it on the next repaint |
-| **Recursive grid**| Transition animation           | ✅                         | ✅                         | ❌                          |
+| **Recursive grid**| Transition animation           | ✅                         | ✅                         | ✅                          |
 | **Recursive grid**| Virtual pointer indicator      | ✅                         | ✅                         | ✅                          |
 | **Recursive grid**| Sub-key preview                | ✅ mini-grid of next keys  | ✅ mini-grid of next keys  | ✅ mini-grid of next keys   |
 | **Scroll**        | Smooth scroll animation        | ✅                         | ✅ (X11: whole notches)    | ❌                          |
@@ -1078,8 +1077,6 @@ green in every cell while an option means nothing, which is exactly how
 | `hints.vision.rectangle_min_size` | option | ✅ | ❌ | ❌ | rectangle detection has no OCR answer, so it stays macOS-only even where the vision strategy lands; that half is text-only |
 | `hints.vision.rectangle_min_aspect` | option | ✅ | ❌ | ❌ | rectangle detection has no OCR answer, so it stays macOS-only even where the vision strategy lands; that half is text-only |
 | `hints.vision.rectangle_max_aspect` | option | ✅ | ❌ | ❌ | rectangle detection has no OCR answer, so it stays macOS-only even where the vision strategy lands; that half is text-only |
-| `recursive_grid.animation.enabled` | option | ✅ | ✅ | ❌ | the Windows overlay backend has no grid transition animation |
-| `recursive_grid.animation.duration_ms` | option | ✅ | ✅ | ❌ | the Windows overlay backend has no grid transition animation |
 | `smooth_cursor.move_mouse_enabled` | option | ✅ | ✅ | ❌ | cursor movement is not animated on Windows |
 | `smooth_cursor.steps` | option | ✅ | ✅ | ❌ | cursor movement is not animated on Windows |
 | `smooth_cursor.max_duration` | option | ✅ | ✅ | ❌ | cursor movement is not animated on Windows |
@@ -1186,12 +1183,11 @@ working, which is exactly why the build exists.
 **Windows**
 
 1. Native notifications — no toast support
-2. Grid and recursive-grid transition animation — not implemented
-3. Smooth cursor and smooth scroll animation — not implemented
-4. Font resolution — alias mapping only, no system font enumeration
-5. `neru services` — every subcommand returns `CodeNotSupported`, where macOS
+2. Smooth cursor and smooth scroll animation — not implemented
+3. Font resolution — alias mapping only, no system font enumeration
+4. `neru services` — every subcommand returns `CodeNotSupported`, where macOS
    installs a launchd agent and Linux a systemd user unit
-6. IPC endpoint, client side — the daemon's endpoint is scoped to one user on
+5. IPC endpoint, client side — the daemon's endpoint is scoped to one user on
     every platform, but only the Unix client checks that for itself before
     connecting. A named pipe carries no ownership a client can read without
     opening it, so the Windows CLI trusts the name it derives from its own SID.

internal/adapter/overlay/AGENTS.md

@@ -3,7 +3,7 @@
 These rules fail silently at runtime, not at compile time; read before editing.
 
 - **Threading is platform-asymmetric.** macOS serializes through the Obj-C bridge (`dispatch_async` to the main thread); Linux must serialize itself — Cairo/X11/Wayland calls are not thread-safe (`linux/manager.go`).
-- **A draw may block, and must never be called while the mode handler's lock is held.** Draws dispatch asynchronously on macOS but hold `renderMu` synchronously on Linux; that asymmetry is deliberate, so callers must assume the blocking case. Windows sits between: a draw holds `renderMu` while it queues commands, and `Flush` hands the frame to the overlay UI thread and returns — painting and presenting happen there, and frames that pile up coalesce (`platform/windows/overlay.go`), so nothing on the keyboard hook's thread waits for pixels. The mode handler computes what to draw under its lock and draws after releasing it (`internal/app/modes/AGENTS.md`); the exceptions are the whole hints surface — the update callback (`hintdraw.go`), the theme refresh and the search input, which now go through the port but still draw under the handler lock, and whose first draw per activation runs the entire `ShowFrame` transition (resize, show, switch) under it — both grid surfaces, whose activations, redraws and per-keystroke updates all run under it (#1211) — and mode teardown, which hides indicators and clears the frame under it. `h.mu` → `renderMu` is therefore a real edge, and safe only while the reverse never exists: nothing holding `renderMu` may call into the app layer or publish to a subscriber that takes `h.mu`.
+- **A draw may block, and must never be called while the mode handler's lock is held.** Draws dispatch asynchronously on macOS but hold `renderMu` synchronously on Linux; that asymmetry is deliberate, so callers must assume the blocking case. Windows sits between: a draw holds `renderMu` while it queues commands, and `Flush` hands the frame to the overlay UI thread and returns — painting and presenting happen there, and frames that pile up coalesce (`platform/windows/overlay.go`), so nothing on the keyboard hook's thread waits for pixels; the recursive-grid transition there is a goroutine that takes `renderMu` per frame and hands each one to that thread the same way (`windows/transition.go`), and every draw that repaints the surface cancels it under the lock first. The mode handler computes what to draw under its lock and draws after releasing it (`internal/app/modes/AGENTS.md`); the exceptions are the whole hints surface — the update callback (`hintdraw.go`), the theme refresh and the search input, which now go through the port but still draw under the handler lock, and whose first draw per activation runs the entire `ShowFrame` transition (resize, show, switch) under it — both grid surfaces, whose activations, redraws and per-keystroke updates all run under it (#1211) — and mode teardown, which hides indicators and clears the frame under it. `h.mu` → `renderMu` is therefore a real edge, and safe only while the reverse never exists: nothing holding `renderMu` may call into the app layer or publish to a subscriber that takes `h.mu`.
 - **Lock topology is deliberate.** The manager owns `renderMu`, held across synchronous draws; animation goroutines lock it via `sharedOverlay`. The mouse-action indicator owns an independent X11/Wayland connection and must **not** share `renderMu` — it has its own `indicatorMu` / `indicatorRenderMu`. **Canceling an animation is the one thing that happens outside it** (#1490): `cancelAnimation` waits for a goroutine that takes `renderMu` on every frame, so every Linux manager method that stops one cancels *before* it takes the lock — and then re-reads the backend pointer under it, because the gap is where a `Destroy` lands. The corollary is that no repaint inside a backend may reach `clear()`, whose first act is that cancel; the ones that clear a surface they are about to redraw go through the `surfaceClear` primitive instead.
 - **Surface primitives split** (#1177): layout, animation, offsets, and label logic live once on `sharedOverlay`; only buffer management, HiDPI scale, and window lifecycle go behind `overlaySurface`. Shared code never touches cgo — primitives take Go types and own their C marshaling, including CString lifetimes.
 - **The Linux backends' exported methods live on `sharedOverlay`, and the manager's nil check is what makes calling one safe** (#1415, ADR 0010): eighteen of them — every draw plus `Hide`, `Clear`, `ClearRect`, `Flush`, `SetHideUnmatched`, `HideHintSearchInput`, `setOriginOffset`, and the pair grid mode's pointer stand-in travels on, `SetGridPointer` / `forgetGridPointer` (#1463) — are declared once and promoted into `x11Overlay` / `wlrootsOverlay`. Each guards itself with `sharedOverlay.drawable()` — is a surface wired, and does `alive()`, the question `overlaySurface` now declares and each backend answers against its own `raw`, still say the native handle is open — in place of the `o.raw != nil` prologue it used to carry. Only `Show`, `Resize` and `Destroy` stay per-backend, because only those three genuinely differ. A promoted method reached through a **nil** backend pointer panics on the promotion, before any receiver guard inside could run — so every dispatch in `linux/manager.go` nil-checks the pointer first, and reads it once (`cancelBackendAnimation`) rather than twice. Those checks are not an interface nobody wrote; ADR 0010 is why, and deleting one turns a silent no-op into a crash. The `!cgo` twins deliberately keep their methods per-backend: their constructors always return nil, so every body exists precisely to be reached on a nil receiver. `Get()` must never return a typed nil either, or every `!= nil` guard downstream silently passes (`backend_linux.go`).

internal/adapter/overlay/linux/animation_cgo.go

@@ -6,6 +6,8 @@ import (
 	"image"
 	"math"
 	"time"
+
+	"github.com/y3owk1n/neru/internal/adapter/overlay/render/motion"
 )
 
 const (
@@ -34,30 +36,6 @@ const (
 	alphaRoundBias        = 0.5
 )
 
-// easeInOut applies a smoothstep ease-in-out interpolation.
-// Matches the visual feel of kCAMediaTimingFunctionEaseInEaseOut on macOS.
-func easeInOut(progress float64) float64 {
-	const (
-		smoothStep3 = 3
-		smoothStep2 = 2
-	)
-
-	if progress <= 0 {
-		return 0
-	}
-
-	if progress >= 1 {
-		return 1
-	}
-
-	return progress * progress * (smoothStep3 - smoothStep2*progress)
-}
-
-// lerp linearly interpolates between a and b by t.
-func lerp(a, b, t float64) float64 {
-	return a + (b-a)*t
-}
-
 // applyEasing maps a linear progress in [0,1] through the named easing curve,
 // matching the easing names accepted by the mouse-action-indicator config
 // (linear, ease_in, ease_out, ease_in_out). Unknown names fall back to
@@ -77,7 +55,7 @@ func applyEasing(easing string, progress float64) float64 {
 	case easingEaseIn:
 		return progress * progress * progress
 	case easingEaseInOut:
-		return easeInOut(progress)
+		return motion.EaseInOut(progress)
 	case easingEaseOut:
 		// Computed below, shared with the unknown-name fallback.
 	}

internal/adapter/overlay/linux/overlay_shared_cgo.go

@@ -12,6 +12,7 @@ import (
 	"github.com/y3owk1n/neru/internal/adapter/overlay/render/badge"
 	gridcomponent "github.com/y3owk1n/neru/internal/adapter/overlay/render/grid"
 	hintscomponent "github.com/y3owk1n/neru/internal/adapter/overlay/render/hints"
+	"github.com/y3owk1n/neru/internal/adapter/overlay/render/motion"
 	recursivegridcomponent "github.com/y3owk1n/neru/internal/adapter/overlay/render/recursivegrid"
 	"github.com/y3owk1n/neru/internal/domain"
 	domainGrid "github.com/y3owk1n/neru/internal/domain/grid"
@@ -149,6 +150,14 @@ type sharedOverlay struct {
 	lastDepth        int
 	lastRects        []image.Rectangle
 	currentAnimRects []image.Rectangle
+	// animSettled says the transition that painted currentAnimRects reached
+	// its last frame; one that has not is continued rather than restarted.
+	animSettled bool
+	// animPointer is where the last transition frame painted the virtual
+	// pointer, and lastPointer where the last settled frame did: the pointer
+	// rides the zoom from one of them to where the new frame puts it.
+	animPointer image.Point
+	lastPointer recursivegridcomponent.VirtualPointerState
 }
 
 // The exported methods below are what the manager calls on a backend. They
@@ -564,7 +573,12 @@ func (o *sharedOverlay) drawRecursiveGridWithSubKeyPreview(
 			duration = 50 * time.Millisecond
 		}
 
+		continuing := len(o.currentAnimRects) > 0 && !o.animSettled
 		fromRects := o.buildFromRects(cellRects, bounds)
+		fromPointer := motion.PointerOrigin(
+			virtualPointer.Position, o.lastPointer.Position, o.animPointer,
+			o.lastPointer.Visible, continuing,
+		)
 		keyRunes := []rune(strings.ToUpper(keys))
 		nextKeyRunes := []rune(strings.ToUpper(nextKeys))
 
@@ -577,7 +591,7 @@ func (o *sharedOverlay) drawRecursiveGridWithSubKeyPreview(
 			fromRects, cellRects,
 			keyRunes, nextKeyRunes,
 			nextDims,
-			style, virtualPointer,
+			style, virtualPointer, fromPointer, continuing,
 			duration, animStop, animDone,
 		)
 	} else {
@@ -591,6 +605,7 @@ func (o *sharedOverlay) drawRecursiveGridWithSubKeyPreview(
 	o.hasLast = true
 	o.lastBounds = bounds
 	o.lastDepth = depth
+	o.lastPointer = virtualPointer
 	o.lastRects = make([]image.Rectangle, len(cellRects))
 	copy(o.lastRects, cellRects)
 }
@@ -899,8 +914,8 @@ func (o *sharedOverlay) startMouseActionAnimation(
 		}
 
 		eased := applyEasing(style.Easing, rawProgress)
-		scale := max(lerp(style.StartScale, style.EndScale, eased), 0)
-		opacity := lerp(style.StartOpacity, style.EndOpacity, eased)
+		scale := max(motion.Lerp(style.StartScale, style.EndScale, eased), 0)
+		opacity := motion.Lerp(style.StartOpacity, style.EndOpacity, eased)
 		diameter := baseSize * scale
 		rect := mouseActionIndicatorRect(point, diameter)
 		fill := applyOpacity(fillBase, opacity)
@@ -1027,57 +1042,14 @@ func (o *sharedOverlay) cancelAnimation() {
 	}
 }
 
-//nolint:mnd,varnamelen
+// buildFromRects answers where each cell of the new depth starts its zoom
+// (motion.TransitionOrigins), from the cells a running transition was
+// interrupted on or the ones the last depth drew.
 func (o *sharedOverlay) buildFromRects(
 	toRects []image.Rectangle,
 	bounds image.Rectangle,
 ) []image.Rectangle {
-	if len(o.currentAnimRects) == len(toRects) {
-		from := make([]image.Rectangle, len(o.currentAnimRects))
-		copy(from, o.currentAnimRects)
-
-		return from
-	}
-
-	if len(o.lastRects) == len(toRects) {
-		from := make([]image.Rectangle, len(o.lastRects))
-		copy(from, o.lastRects)
-
-		return from
-	}
-
-	if o.lastBounds.Empty() {
-		from := make([]image.Rectangle, len(toRects))
-		for idx, rect := range toRects {
-			cx := rect.Min.X + rect.Dx()/2
-			cy := rect.Min.Y + rect.Dy()/2
-			from[idx] = image.Rect(cx, cy, cx, cy)
-		}
-
-		return from
-	}
-
-	fromBounds := o.lastBounds
-	fw := float64(fromBounds.Dx())
-	fh := float64(fromBounds.Dy())
-	dw := float64(bounds.Dx())
-	dh := float64(bounds.Dy())
-
-	from := make([]image.Rectangle, len(toRects))
-	for idx, rect := range toRects {
-		nx := (float64(rect.Min.X+rect.Dx()/2) - float64(bounds.Min.X)) / dw
-		ny := (float64(rect.Min.Y+rect.Dy()/2) - float64(bounds.Min.Y)) / dh
-		cx := int(float64(fromBounds.Min.X) + nx*fw)
-		cy := int(float64(fromBounds.Min.Y) + ny*fh)
-		rw := rect.Dx()
-		rh := rect.Dy()
-		from[idx] = image.Rect(
-			cx-rw/2, cy-rh/2,
-			cx+rw/2, cy+rh/2,
-		)
-	}
-
-	return from
+	return motion.TransitionOrigins(toRects, bounds, o.currentAnimRects, o.lastRects, o.lastBounds)
 }
 
 func (o *sharedOverlay) startGridAnimation(
@@ -1086,38 +1058,35 @@ func (o *sharedOverlay) startGridAnimation(
 	nextDims domain.GridDimensions,
 	style recursivegridcomponent.Style,
 	virtualPointer recursivegridcomponent.VirtualPointerState,
+	fromPointer image.Point,
+	continuing bool,
 	duration time.Duration,
 	stopCh chan struct{},
 	doneCh chan struct{},
 ) {
 	o.srf.syncBeforeAnimation()
 
 	startTime := time.Now()
+	o.animSettled = false
 
-	renderFrame := func(rawProgress float64) {
-		if rawProgress >= 1.0 {
-			rawProgress = 1.0
-		}
+	// Called under renderMu. The progress is read here rather than before the
+	// lock was taken, so a frame that waited on it paints where the cells are
+	// now rather than where they were when it was scheduled.
+	renderFrame := func() float64 {
+		rawProgress := min(float64(time.Since(startTime))/float64(duration), 1)
+		progress := motion.Eased(rawProgress, continuing)
 
-		progress := easeInOut(rawProgress)
-
-		interpCells := make([]image.Rectangle, len(toRects))
-		for i := range toRects {
-			src := fromRects[i]
-			dst := toRects[i]
-			interpCells[i] = image.Rect(
-				int(lerp(float64(src.Min.X), float64(dst.Min.X), progress)),
-				int(lerp(float64(src.Min.Y), float64(dst.Min.Y), progress)),
-				int(lerp(float64(src.Max.X), float64(dst.Max.X), progress)),
-				int(lerp(float64(src.Max.Y), float64(dst.Max.Y), progress)),
-			)
-		}
+		interpCells := motion.LerpRects(fromRects, toRects, progress)
+		pointer := virtualPointer
+		pointer.Position = motion.LerpPoint(fromPointer, virtualPointer.Position, progress)
 
 		if !o.srf.beginFrame() {
-			return
+			return rawProgress
 		}
 
 		o.currentAnimRects = interpCells
+		o.animPointer = pointer.Position
+		o.animSettled = rawProgress >= 1
 
 		o.srf.clearFrame()
 		o.drawFrame(
@@ -1126,8 +1095,10 @@ func (o *sharedOverlay) startGridAnimation(
 			nextKeyRunes,
 			nextDims,
 			style,
-			virtualPointer,
+			pointer,
 		)
+
+		return rawProgress
 	}
 
 	go func() {
@@ -1151,13 +1122,6 @@ func (o *sharedOverlay) startGridAnimation(
 			default:
 			}
 
-			elapsed := time.Since(startTime)
-
-			rawProgress := float64(elapsed) / float64(duration)
-			if rawProgress >= 1.0 {
-				rawProgress = 1.0
-			}
-
 			renderStart := time.Now()
 
 			renderMu := o.renderMu
@@ -1176,13 +1140,13 @@ func (o *sharedOverlay) startGridAnimation(
 				}
 			}
 
-			renderFrame(rawProgress)
+			rawProgress := renderFrame()
 
 			if renderMu != nil {
 				renderMu.Unlock()
 			}
 
-			if rawProgress >= 1.0 {
+			if rawProgress >= 1 {
 				return
 			}