Backed out changeset 85a9eb44b240 (bug 366791) due to bug 449221.

layout/xul/base/src/nsScrollbarFrame.cpp

@@ -66,6 +66,23 @@ NS_INTERFACE_MAP_BEGIN(nsScrollbarFrame)
   NS_INTERFACE_MAP_ENTRY(nsIScrollbarFrame)
 NS_INTERFACE_MAP_END_INHERITING(nsBoxFrame)
 
+
+NS_IMETHODIMP
+nsScrollbarFrame::Init(nsIContent* aContent,
+                       nsIFrame*   aParent,
+                       nsIFrame*   aPrevInFlow)
+{
+  nsresult  rv = nsBoxFrame::Init(aContent, aParent, aPrevInFlow);
+
+  // We want to be a reflow root since we use reflows to move the
+  // slider.  Any reflow inside the scrollbar frame will be a reflow to
+  // move the slider and will thus not change anything outside of the
+  // scrollbar or change the size of the scrollbar frame.
+  mState |= NS_FRAME_REFLOW_ROOT;
+
+  return rv;
+}
+
 NS_IMETHODIMP
 nsScrollbarFrame::Reflow(nsPresContext*          aPresContext,
                          nsHTMLReflowMetrics&     aDesiredSize,

layout/xul/base/src/nsScrollbarFrame.h

@@ -88,6 +88,10 @@ public:
                            nsGUIEvent *    aEvent,
                            nsEventStatus*  aEventStatus);
 
+  NS_IMETHOD Init(nsIContent*      aContent,
+                  nsIFrame*        aParent,
+                  nsIFrame*        aPrevInFlow);
+
   NS_IMETHOD Reflow(nsPresContext*          aPresContext,
                     nsHTMLReflowMetrics&     aDesiredSize,
                     const nsHTMLReflowState& aReflowState,