0.10.4: button still-frames render only on their own state

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/lib/interpolate.ts

@@ -174,6 +174,42 @@ export function isLayerActiveAt(layer: Layer, time: number): boolean {
   return time >= range.min - EPSILON && time <= range.max + EPSILON
 }
 
+/**
+ * Visibility gate for layers INSIDE a button asset. The 4 discrete
+ * state times (normal / hover / pressed / disabled) are treated as
+ * exact frames — a still-frame (hold) keyframe renders only on its own
+ * state and is invisible on neighbours, not "held forward until the
+ * next keyframe" as on a regular timeline.
+ *
+ * Rules:
+ *  - No keyframes → static from base, always visible.
+ *  - `time` matches a keyframe's time → visible.
+ *  - `time` sits strictly between two adjacent keyframes → visible
+ *    only when the left bracket is NOT a hold (i.e. an interpolation
+ *    region, not a held-into-silence region).
+ *  - Before the first keyframe or after the last → hidden.
+ */
+export function isButtonLayerActiveAt(layer: Layer, time: number): boolean {
+  const EPSILON = 1e-6
+  const kfs: Keyframe[] = []
+  for (const k of layer.keyframes) kfs.push(k)
+  if (layer.positionKeyframes) for (const k of layer.positionKeyframes) kfs.push(k)
+  if (layer.rotationKeyframes) for (const k of layer.rotationKeyframes) kfs.push(k)
+  if (kfs.length === 0) return true
+  kfs.sort((a, b) => a.time - b.time)
+  for (const kf of kfs) {
+    if (Math.abs(kf.time - time) < EPSILON) return true
+  }
+  for (let i = 0; i < kfs.length - 1; i++) {
+    const a = kfs[i]
+    const b = kfs[i + 1]
+    if (time > a.time + EPSILON && time < b.time - EPSILON) {
+      return !a.hold
+    }
+  }
+  return false
+}
+
 export function sampleLayer(layer: Layer, time: number): AnimatableProps {
   const main = sampleKeyframeSet(layer.keyframes, layer.base, time)
   if (!layer.separated) return main