Bug 623432 part 2 - Ignore scrollbars while drag-selecting content. r=tn

layout/generic/nsFrame.cpp

@@ -3553,17 +3553,17 @@ NS_IMETHODIMP nsFrame::HandleDrag(nsPresContext* aPresContext,
   MOZ_ASSERT(aEvent->mClass == eMouseEventClass,
              "HandleDrag can only handle mouse event");
 
-  bool selectable;
-  IsSelectable(&selectable, nullptr);
-
+  nsIFrame* scrollbar =
+    nsLayoutUtils::GetClosestFrameOfType(this, nsGkAtoms::scrollbarFrame);
+  if (!scrollbar) {
     // XXX Do we really need to exclude non-selectable content here?
     // GetContentOffsetsFromPoint can handle it just fine, although some
     // other stuff might not like it.
-  if (!selectable)
-    return NS_OK;
+    // NOTE: DisplaySelection() returns SELECTION_OFF for non-selectable frames.
     if (DisplaySelection(aPresContext) == nsISelectionController::SELECTION_OFF) {
       return NS_OK;
     }
+  }
   nsIPresShell *presShell = aPresContext->PresShell();
 
   RefPtr<nsFrameSelection> frameselection = GetFrameSelection();