Bug 1093949 - Reverse scroll position for RTL content. r=mats

layout/generic/nsGfxScrollFrame.cpp

@@ -1132,7 +1132,7 @@ ScrollFrameHelper::ThumbMoved(nsScrollbarFrame* aScrollbar,
   nsPoint current = GetScrollPosition();
   nsPoint dest = current;
   if (isHorizontal) {
-    dest.x = aNewPos;
+    dest.x = IsLTR() ? aNewPos : aNewPos - GetScrollRange().width;
   } else {
     dest.y = aNewPos;
   }

layout/xul/nsIScrollbarMediator.h

@@ -41,6 +41,9 @@ public:
   virtual void RepeatButtonScroll(nsScrollbarFrame* aScrollbar) = 0;
   /**
    * aOldPos and aNewPos are scroll positions.
+   * The scroll positions start with zero at the left edge; implementors that want
+   * zero at the right edge for RTL content will need to adjust accordingly.
+   * (See ScrollFrameHelper::ThumbMoved in nsGfxScrollFrame.cpp.)
    * @note This method might destroy the frame, pres shell, and other objects.
    */
   virtual void ThumbMoved(nsScrollbarFrame* aScrollbar,