Use an up-to-date canvas size when building canvas filters

third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp

Index: third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp
diff --git a/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp b/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp
index 8cf6c194b1d8705f3a361699351866cedf243c06..819b6ffdfdb27598838574573ba85da2bc71ba84 100644
--- a/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp
+++ b/third_party/WebKit/Source/modules/canvas2d/CanvasRenderingContext2D.cpp
@@ -876,6 +876,13 @@ bool CanvasRenderingContext2D::draw(const DrawFunc& drawFunc, const ContainsFunc
     if (!state().isTransformInvertible())
         return false;
 
+    // Filter construction can depend on the bounding box of the canvas' layout object,
+    // so we need layout to be up-to-date if filter construction is needed. However,
+    // layout can trigger scripts that cause the canvas to be freed, so we need to
+    // trigger layout before checking for the drawing canvas.
+    if (state().needsToBuildFilter())
+        canvas()->document().updateLayoutIgnorePendingStylesheets();

[Justin Novosad, 2016/01/15 21:16:28]

This is a big hammer that may affect performance if we call it when not
necessary. You can make the layout conditional on the canvas's style being
dirty. Basically you want to avoid recomputing style when the style recalc would
only affect objects that are not in the canvas's hierarchy. Use this instead:
canvas()->document().updateLayoutTreeForNodeIfNeeded(canvas());

[ajuma, 2016/01/18 14:34:15]

Turns out that updateLayoutTreeForNodeIfNeeded isn't sufficient (at least not
for the test case); document()->needsLayoutTreeUpdate() is false, so
updateLayoutTreeForNodeIfNeeded() earlies-out without doing any useful work. But
updateLayoutIgnorePendingStylesheets works because the FrameView needs layout
(since it hasn't done any layout yet -- FrameView::didFirstLayout is false at
this point), and the call to FrameView::layout leads to
FrameView::layoutFromRootObject which does layout on the whole tree.

Maybe a better approach is to pass the canvas rect directly to the filter
builder rather than having the builder retrieve the rect from the layout tree.
Let me see if that works.

+
     SkIRect clipBounds;
     if (!drawingCanvas() || !drawingCanvas()->getClipDeviceBounds(&clipBounds))
         return false;