Fixing color bleeding when the canvas is stretched.

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 2a5a8ff5939da3b58adbbd60f3f5670202277781..b87d3fb7f4ac373d4b0e7cb4fb155d772443c00d 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -323,12 +323,22 @@ void HTMLCanvasElement::didFinalizeFrame() {
   // Propagate the m_dirtyRect accumulated so far to the compositor
   // before restarting with a blank dirty rect.
   FloatRect srcRect(0, 0, size().width(), size().height());
-  m_dirtyRect.intersect(srcRect);
+
   LayoutBox* ro = layoutBox();
   // Canvas content updates do not need to be propagated as
   // paint invalidations if the canvas is accelerated, since
   // the canvas contents are sent separately through a texture layer.
   if (ro && (!m_context || !m_context->isAccelerated())) {
+    // If ro->contentBoxRect() is larger than srcRect the canvas's image is
+    // being stretched, so we need to account for color bleeding caused by the
+    // interpollation filter.
+    if (FloatRect(ro->contentBoxRect()).contains(srcRect) &&

[Justin Novosad, 2016/10/21 15:56:03]

"Contains" is not the right condition to determine whether the image is being
stretched. You could be stretching in one dimension and compressing in another,
there could also be a position offset due to padding.  What you should do here
is compare the width and height of the rects.

+        srcRect != FloatRect(ro->contentBoxRect())) {
+      m_dirtyRect.expand(FloatSize(0.5, 0.5));

[Justin Novosad, 2016/10/21 15:56:03]

You should use inflate here.

+      m_dirtyRect.move(FloatSize(-0.25, -0.25));
+    }
+
+    m_dirtyRect.intersect(srcRect);
     LayoutRect mappedDirtyRect(enclosingIntRect(
         mapRect(m_dirtyRect, srcRect, FloatRect(ro->contentBoxRect()))));
     // For querying PaintLayer::compositingState()