Optimize static content scrolling

lib/web_ui/lib/src/engine/bitmap_canvas.dart

@@ -545,7 +545,7 @@ class BitmapCanvas extends EngineCanvas {
   /// Paints the [picture] into this canvas.
   void drawPicture(ui.Picture picture) {
     final EnginePicture enginePicture = picture;
-    enginePicture.recordingCanvas.apply(this);
+    enginePicture.recordingCanvas.apply(this, bounds);
   }
 
   /// Draws vertices on a gl context.

lib/web_ui/lib/src/engine/picture.dart

@@ -32,6 +32,7 @@ class EnginePictureRecorder implements ui.PictureRecorder {
       return null;
     }
     _isRecording = false;
+    _canvas.endRecording();
     return EnginePicture(_canvas, cullRect);
   }
 }
@@ -46,8 +47,9 @@ class EnginePicture implements ui.Picture {
 
   @override
   Future<ui.Image> toImage(int width, int height) async {
-    final BitmapCanvas canvas = BitmapCanvas(ui.Rect.fromLTRB(0, 0, width.toDouble(), height.toDouble()));
-    recordingCanvas.apply(canvas);
+    final ui.Rect imageRect = ui.Rect.fromLTRB(0, 0, width.toDouble(), height.toDouble());
+    final BitmapCanvas canvas = BitmapCanvas(imageRect);
+    recordingCanvas.apply(canvas, imageRect);
     final String imageDataUrl = canvas.toDataUrl();
     final html.ImageElement imageElement = html.ImageElement()
       ..src = imageDataUrl

lib/web_ui/lib/src/engine/surface/picture.dart

@@ -145,7 +145,7 @@ class PersistedHoudiniPicture extends PersistedPicture {
     _canvas = canvas;
     domRenderer.clearDom(rootElement);
     rootElement.append(_canvas.rootElement);
-    picture.recordingCanvas.apply(_canvas);
+    picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
     canvas.commit();
   }
 }
@@ -231,7 +231,7 @@ class PersistedStandardPicture extends PersistedPicture {
     _canvas = DomCanvas();
     domRenderer.clearDom(rootElement);
     rootElement.append(_canvas.rootElement);
-    picture.recordingCanvas.apply(_canvas);
+    picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
   }
 
   void _applyBitmapPaint(EngineCanvas oldCanvas) {
@@ -244,7 +244,7 @@ class PersistedStandardPicture extends PersistedPicture {
       oldCanvas.bounds = _optimalLocalCullRect;
       _canvas = oldCanvas;
       _canvas.clear();
-      picture.recordingCanvas.apply(_canvas);
+      picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
     } else {
       // We can't use the old canvas because the size has changed, so we put
       // it in a cache for later reuse.
@@ -265,7 +265,7 @@ class PersistedStandardPicture extends PersistedPicture {
           domRenderer.clearDom(rootElement);
           rootElement.append(_canvas.rootElement);
           _canvas.clear();
-          picture.recordingCanvas.apply(_canvas);
+          picture.recordingCanvas.apply(_canvas, _optimalLocalCullRect);
         },
       ));
     }
@@ -352,7 +352,7 @@ class PersistedStandardPicture extends PersistedPicture {
 /// to draw shapes and text.
 abstract class PersistedPicture extends PersistedLeafSurface {
   PersistedPicture(this.dx, this.dy, this.picture, this.hints)
-      : localPaintBounds = picture.recordingCanvas.computePaintBounds();
+      : localPaintBounds = picture.recordingCanvas.pictureBounds;
 
   EngineCanvas _canvas;
 
@@ -491,7 +491,7 @@ abstract class PersistedPicture extends PersistedLeafSurface {
 
     // The new cull rect contains area not covered by a previous rect. Perhaps
     // the clip is growing, moving around the picture, or both. In this case
-    // a part of the picture may not been painted. We will need to
+    // a part of the picture may not have been painted. We will need to
     // request a new canvas and paint the picture on it. However, this is also
     // a strong signal that the clip will continue growing as typically
     // Flutter uses animated transitions. So instead of allocating the canvas
@@ -500,32 +500,45 @@ abstract class PersistedPicture extends PersistedLeafSurface {
     // will hit the above case where the new cull rect is fully contained
     // within the cull rect we compute now.
 
-    // If any of the borders moved.
-    // TODO(yjbanov): consider switching to Mouad's snap-to-10px strategy. It
-    //                might be sufficient, if not more effective.
-    const double kPredictedGrowthFactor = 3.0;
-    final double leftwardTrend = kPredictedGrowthFactor *
-        math.max(oldOptimalLocalCullRect.left - _exactLocalCullRect.left, 0);
-    final double upwardTrend = kPredictedGrowthFactor *
-        math.max(oldOptimalLocalCullRect.top - _exactLocalCullRect.top, 0);
-    final double rightwardTrend = kPredictedGrowthFactor *
-        math.max(_exactLocalCullRect.right - oldOptimalLocalCullRect.right, 0);
-    final double bottomwardTrend = kPredictedGrowthFactor *
-        math.max(
-            _exactLocalCullRect.bottom - oldOptimalLocalCullRect.bottom, 0);
+    // Compute the delta, by which each of the side of the clip rect has "moved"
+    // since the last time we updated the cull rect.
+    final double leftwardDelta = oldOptimalLocalCullRect.left - _exactLocalCullRect.left;
+    final double upwardDelta = oldOptimalLocalCullRect.top - _exactLocalCullRect.top;
+    final double rightwardDelta = _exactLocalCullRect.right - oldOptimalLocalCullRect.right;
+    final double bottomwardDelta = _exactLocalCullRect.bottom - oldOptimalLocalCullRect.bottom;
 
+    // Compute the new optimal rect to paint into.
     final ui.Rect newLocalCullRect = ui.Rect.fromLTRB(
-      oldOptimalLocalCullRect.left - leftwardTrend,
-      oldOptimalLocalCullRect.top - upwardTrend,
-      oldOptimalLocalCullRect.right + rightwardTrend,
-      oldOptimalLocalCullRect.bottom + bottomwardTrend,
+      _exactLocalCullRect.left - _predictTrend(leftwardDelta, _exactLocalCullRect.width),
+      _exactLocalCullRect.top - _predictTrend(upwardDelta, _exactLocalCullRect.height),
+      _exactLocalCullRect.right + _predictTrend(rightwardDelta, _exactLocalCullRect.width),
+      _exactLocalCullRect.bottom + _predictTrend(bottomwardDelta, _exactLocalCullRect.height),
     ).intersect(localPaintBounds);
 
     final bool localCullRectChanged = _optimalLocalCullRect != newLocalCullRect;
     _optimalLocalCullRect = newLocalCullRect;
     return localCullRectChanged;
   }
 
+  /// Predicts the delta a particular side of a clip rect will move given the
+  /// [delta] it moved by last, and the respective [extent] (width or height)
+  /// of the clip.
+  static double _predictTrend(double delta, double extent) {
+    if (delta <= 0.0) {
+      // Shrinking. Give it 10% of the extent in case the trend is reversed.
+      return extent * 0.1;
+    } else {
+      // Growing. Predict 10 more frames of similar deltas. Give it at least
+      // 50% of the extent (protect from extremely slow growth trend such as
+      // slow scrolling). Give no more than the full extent (protects from
+      // fast scrolling that could lead to overallocation).
+      return math.min(
+        math.max(extent * 0.5, delta * 10.0),
+        extent,
+      );
+    }
+  }
+
   /// Number of bitmap pixel painted by this picture.
   ///
   /// If the implementation does not paint onto a bitmap canvas, it should