Fixing 2D canvas invalidation region tracking.

Source/core/html/HTMLCanvasElement.cpp

Index: Source/core/html/HTMLCanvasElement.cpp
diff --git a/Source/core/html/HTMLCanvasElement.cpp b/Source/core/html/HTMLCanvasElement.cpp
index a8da6d7bd3d426c8ac8151fbf8e95ada0009ebc2..2402c38c720413af67e5f7ef54ead5eb40ff9f05 100644
--- a/Source/core/html/HTMLCanvasElement.cpp
+++ b/Source/core/html/HTMLCanvasElement.cpp
@@ -195,19 +195,26 @@ void HTMLCanvasElement::didDraw(const FloatRect& rect)
     clearCopiedImage();
 
     if (RenderBox* ro = renderBox()) {
-        FloatRect destRect = ro->contentBoxRect();
-        FloatRect r = mapRect(rect, FloatRect(0, 0, size().width(), size().height()), destRect);
-        r.intersect(destRect);
+        FloatRect srcRect(0, 0, size().width(), size().height());
+        FloatRect r = rect;
+        r.intersect(srcRect);
         if (r.isEmpty() || m_dirtyRect.contains(r))
             return;
-
         m_dirtyRect.unite(r);
-        ro->invalidatePaintRectangle(enclosingIntRect(m_dirtyRect));
+        FloatRect mappedDirtyRect = mapRect(r, srcRect, ro->contentBoxRect());
+
+        ro->invalidatePaintRectangle(enclosingIntRect(mappedDirtyRect));
     }
 
     notifyObserversCanvasChanged(rect);
 }
 
+void HTMLCanvasElement::didPresent()
+{
+    // Canvas was presented externally (without going through paint())
+    m_dirtyRect = FloatRect();
+}
+
 void HTMLCanvasElement::notifyObserversCanvasChanged(const FloatRect& rect)
 {
     WillBeHeapHashSet<RawPtrWillBeWeakMember<CanvasObserver> >::iterator end = m_observers.end();
@@ -220,6 +227,8 @@ void HTMLCanvasElement::reset()
     if (m_ignoreReset)
         return;
 
+    m_dirtyRect = FloatRect();
+
     bool ok;
     bool hadImageBuffer = hasImageBuffer();