fix(canvas): filter retained backing previews

- Keep stale scene backings available during zoom for smooth interaction

- Draw scaled backings with linear filtering to avoid blocky text artifacts

packages/core/src/canvas/renderer/retained-backing.ts

@@ -103,6 +103,10 @@ function backingWorldCoverageContainsLiveViewport(r: SkiaRenderer): boolean {
   )
 }
 
+function backingZoomMatchesLiveViewport(r: SkiaRenderer): boolean {
+  return Math.abs((r.sceneBacking?.zoom ?? r.zoom) - r.zoom) <= 0.0001
+}
+
 function backingCoverageContainsLiveViewport(
   r: SkiaRenderer,
   sceneVersion: number,
@@ -110,7 +114,7 @@ function backingCoverageContainsLiveViewport(
   positionPreviewVersion: number
 ): boolean {
   if (!backingMetadataMatches(r, sceneVersion, positionPreviewVersion)) return false
-  const crispZoom = Math.abs((r.sceneBacking?.zoom ?? r.zoom) - r.zoom) <= 0.0001
+  const crispZoom = backingZoomMatchesLiveViewport(r)
   if (allowStaleZoom && backingScreenCoverageContainsViewport(r)) return true
   return crispZoom && backingWorldCoverageContainsLiveViewport(r)
 }
@@ -139,12 +143,13 @@ function drawSceneBacking(
   const x = r.panX - backing.panX * scale
   const y = r.panY - backing.panY * scale
   r.opacityPaint.setAlphaf(1)
-  canvas.drawImageRect(
+  canvas.drawImageRectOptions(
     backing.image,
     r.ck.LTRBRect(0, 0, backing.width * backing.dpr, backing.height * backing.dpr),
     r.ck.LTRBRect(x, y, x + backing.width * scale, y + backing.height * scale),
-    r.opacityPaint,
-    true
+    r.ck.FilterMode.Linear,
+    r.ck.MipmapMode.None,
+    r.opacityPaint
   )
   return true
 }

tests/engine/render/canvas/retained-backing.test.ts

@@ -17,7 +17,9 @@ function createRenderer(surfaceFactory: () => Surface | null) {
         top,
         right,
         bottom
-      ])
+      ]),
+      FilterMode: { Linear: 'Linear' },
+      MipmapMode: { None: 'None' }
     } as SkiaRenderer['ck'],
     surface: {
       makeSurface: mock(surfaceFactory)
@@ -54,7 +56,8 @@ function createRenderer(surfaceFactory: () => Surface | null) {
 
 function createCanvas() {
   const canvas: Partial<Canvas> = {
-    drawImageRect: mock()
+    drawImageRect: mock(),
+    drawImageRectOptions: mock()
   }
   return canvas as Canvas
 }
@@ -79,10 +82,71 @@ test('retained scene backing falls back when CanvasKit cannot create an offscree
 
   expect(renderSceneBacking(r, canvas, graph, 1)).toBe(false)
   expect(r.surface.makeSurface).toHaveBeenCalled()
-  expect(canvas.drawImageRect).not.toHaveBeenCalled()
+  expect(canvas.drawImageRectOptions).not.toHaveBeenCalled()
   expect(r.sceneBacking).toBeNull()
 })
 
+test('retained scene backing filters cross-zoom previews instead of falling back to live rendering', () => {
+  const r = createRenderer(() => null)
+  r.zoom = 1
+  r.sceneBackingPreviewUntil = Number.POSITIVE_INFINITY
+  r.sceneBacking = {
+    image: { delete: mock() } as CKImage,
+    pageId: 'page',
+    sceneVersion: 1,
+    positionPreviewVersion: 0,
+    panX: 0,
+    panY: 0,
+    zoom: 0.5,
+    width: 300,
+    height: 300,
+    dpr: 1,
+    worldX: 0,
+    worldY: 0,
+    worldWidth: 600,
+    worldHeight: 600
+  } as NonNullable<SkiaRenderer['sceneBacking']>
+  const canvas = createCanvas()
+  const graph = createGraph()
+
+  expect(renderSceneBacking(r, canvas, graph, 1)).toBe(true)
+  expect(canvas.drawImageRectOptions).toHaveBeenCalledWith(
+    r.sceneBacking.image,
+    expect.anything(),
+    expect.anything(),
+    r.ck.FilterMode.Linear,
+    r.ck.MipmapMode.None,
+    r.opacityPaint
+  )
+})
+
+test('retained scene backing allows same-zoom previews while panning', () => {
+  const r = createRenderer(() => null)
+  r.zoom = 1
+  r.sceneBackingPreviewUntil = Number.POSITIVE_INFINITY
+  r.sceneBacking = {
+    image: { delete: mock() } as CKImage,
+    pageId: 'page',
+    sceneVersion: 1,
+    positionPreviewVersion: 0,
+    panX: 0,
+    panY: 0,
+    zoom: 1,
+    width: 300,
+    height: 300,
+    dpr: 1,
+    worldX: 0,
+    worldY: 0,
+    worldWidth: 300,
+    worldHeight: 300
+  } as NonNullable<SkiaRenderer['sceneBacking']>
+  const canvas = createCanvas()
+  const graph = createGraph()
+
+  expect(renderSceneBacking(r, canvas, graph, 1)).toBe(true)
+  expect(canvas.drawImageRectOptions).toHaveBeenCalled()
+})
+
 test('retained scene backing invalidates stale position-preview metadata', () => {
   const r = createRenderer(() => null)
   r.sceneBacking = {
@@ -106,5 +170,5 @@ test('retained scene backing invalidates stale position-preview metadata', () =>
   const graph = createGraph(2)
 
   expect(renderSceneBacking(r, canvas, graph, 1)).toBe(false)
-  expect(canvas.drawImageRect).not.toHaveBeenCalled()
+  expect(canvas.drawImageRectOptions).not.toHaveBeenCalled()
 })