Make scrollbars be reflow roots so that moving the scrollbar in an element with 'overflow' that leads to scrollability doesn't cause a reflow from the top of a document. b=156522 r=waterson sr=kin

layout/xul/base/src/nsScrollbarFrame.cpp

@@ -91,9 +91,24 @@ nsScrollbarFrame::Init(nsIPresContext*  aPresContext,
   nsCOMPtr<nsIViewManager> vm;
   view->GetViewManager(*getter_AddRefs(vm));
   vm->SetViewContentTransparency(view, PR_TRUE);
+
+  // 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::IsPercentageBase(PRBool& aBase) const
+{
+  // Return true so that the nsHTMLReflowState code is happy with us
+  // being a reflow root.
+  aBase = PR_TRUE;
+  return NS_OK;
+}
+
 
 NS_IMETHODIMP
 nsScrollbarFrame::AttributeChanged(nsIPresContext* aPresContext,

layout/xul/base/src/nsScrollbarFrame.h

@@ -96,6 +96,7 @@ public:
               nsIStyleContext* aContext,
               nsIFrame*        aPrevInFlow);
 
+  NS_IMETHOD  IsPercentageBase(PRBool& aBase) const;
 
   // nsIScrollbarFrame
   NS_IMETHOD SetScrollbarMediator(nsIScrollbarMediator* aMediator) { mScrollbarMediator = aMediator; return NS_OK; };