Bug 1378455 - Don't minimize the visible region for the scrollbar thumb in FrameLayerBuilder since it can move asynchronously. r=mstange

layout/painting/FrameLayerBuilder.cpp

@@ -4104,7 +4104,7 @@ ContainerState::ProcessDisplayItems(nsDisplayList* aList)
     if (itemClip.HasClip()) {
       itemContent.IntersectRect(itemContent, itemClip.GetClipRect());
       clipRect = ViewAs<ParentLayerPixel>(ScaleToNearestPixels(itemClip.GetClipRect()));
-      if (!prerenderedTransform) {
+      if (!prerenderedTransform && !IsScrollThumbLayer(item)) {
         itemDrawRect.IntersectRect(itemDrawRect, clipRect.ToUnknownRect());
       }
       clipRect.MoveBy(ViewAs<ParentLayerPixel>(mParameters.mOffset));

layout/xul/nsSliderFrame.cpp

@@ -457,6 +457,9 @@ nsSliderFrame::BuildDisplayListForChildren(nsDisplayListBuilder*   aBuilder,
       // Restore the saved clip so it applies to the thumb container layer.
       thumbContentsClipState.Restore();
 
+      nsDisplayListBuilder::AutoBuildingDisplayList
+        buildingDisplayList(aBuilder, this, dirty, false);
+
       // Wrap the list to make it its own layer.
       const ActiveScrolledRoot* ownLayerASR = contASRTracker.GetContainerASR();
       aLists.Content()->AppendNewToTop(new (aBuilder)