Backed out changeset 6150269410b2 (bug 921212) for bustage on a CLOSED TREE.

gfx/layers/client/ContentClient.cpp

@@ -479,13 +479,21 @@ ContentClientDoubleBuffered::SyncFrontBufferToBackBuffer()
 
   nsIntRegion updateRegion = mFrontUpdatedRegion;
 
+  int32_t xBoundary = mBufferRect.XMost() - mBufferRotation.x;
+  int32_t yBoundary = mBufferRect.YMost() - mBufferRotation.y;
+
+  // Figure out whether the area we want to copy wraps the edges of our buffer.
+  bool needFullCopy = (xBoundary < updateRegion.GetBounds().XMost() &&
+                       xBoundary > updateRegion.GetBounds().x) ||
+                      (yBoundary < updateRegion.GetBounds().YMost() &&
+                       yBoundary > updateRegion.GetBounds().y);
+  
   // This is a tricky trade off, we're going to get stuff out of our
   // frontbuffer now, but the next PaintThebes might throw it all (or mostly)
   // away if the visible region has changed. This is why in reality we want
   // this code integrated with PaintThebes to always do the optimal thing.
 
-  if (mDidSelfCopy) {
-    mDidSelfCopy = false;
+  if (needFullCopy) {
     // We can't easily draw our front buffer into us, since we're going to be
     // copying stuff around anyway it's easiest if we just move our situation
     // to non-rotated while we're at it. If this situation occurs we'll have