Bug 584617. Part 1. Make sure that we don't use the overflow rect of the root frame instead of the dirty region when we are drawing ignoring viewport scrolling because that might be wrong. r=roc a=dbaron

layout/base/nsLayoutUtils.cpp

@@ -1216,7 +1216,8 @@ nsLayoutUtils::PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFra
   nsIPresShell* presShell = presContext->PresShell();
 
   nsRegion visibleRegion;
-  if (aFlags & PAINT_WIDGET_LAYERS) {
+  if ((aFlags & PAINT_WIDGET_LAYERS) &&
+      !(aFlags & PAINT_IGNORE_VIEWPORT_SCROLLING)) {
     // This layer tree will be reused, so we'll need to calculate it
     // for the whole visible area of the window
     visibleRegion = aFrame->GetOverflowRectRelativeToSelf();
@@ -1287,6 +1288,13 @@ nsLayoutUtils::PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFra
     }
   }
 
+  // If we're going to display something different from what we'd normally
+  // paint in a window then we will flush out any retained layer trees before
+  // *and after* we draw.
+  PRBool willFlushLayers = aFlags & (PAINT_IGNORE_SUPPRESSION |
+                                     PAINT_IGNORE_VIEWPORT_SCROLLING |
+                                     PAINT_HIDE_CARET);
+
   nsIAtom* frameType = aFrame->GetType();
   // For the viewport frame in print preview/page layout we want to paint
   // the grey background behind the page, not the canvas color.
@@ -1337,9 +1345,7 @@ nsLayoutUtils::PaintFrame(nsIRenderingContext* aRenderingContext, nsIFrame* aFra
     nsIntRegion visibleWindowRegion(visibleRegion.ToOutsidePixels(pixelRatio));
     nsIntRegion dirtyWindowRegion(aDirtyRegion.ToOutsidePixels(pixelRatio));
 
-    if (aFlags & (PAINT_IGNORE_SUPPRESSION |
-                  PAINT_IGNORE_VIEWPORT_SCROLLING |
-                  PAINT_HIDE_CARET)) {
+    if (willFlushLayers) {
       // We're going to display something different from what we'd normally
       // paint in a window, so make sure we flush out any retained layer
       // trees before *and after* we draw