Bug 1367765 - Do not set the 'active' content state flag on scrollbar thumbs in ActiveElementManager. r=rhunt

Unlike regular content, scrollbar thumbs use a different mechanism for
triggering their 'active' style, and no one will clear this flag if
ActiveElementManager sets it.

MozReview-Commit-ID: AfloVYRkvQA

--HG--
extra : rebase_source : e89fe4eefe615d3f65d2471ff59aa9d51d9ae5e2

gfx/layers/apz/util/ActiveElementManager.cpp

@@ -123,7 +123,12 @@ ActiveElementManager::HandleTouchEndEvent(bool aWasClick)
   // the mouse-down event generated by the click.
   CancelTask();
   if (aWasClick) {
-    SetActive(mTarget);
+    // Scrollbar thumbs use a different mechanism for their active
+    // highlight (the "active" attribute), so don't set the active state
+    // on them because nothing will clear it.
+    if (!(mTarget && mTarget->IsXULElement(nsGkAtoms::thumb))) {
+      SetActive(mTarget);
+    }
   } else {
     // We might reach here if mCanBePan was false on touch-start and
     // so we set the element active right away. Now it turns out the