Bug 1511358 - Don't include elements with z-index<0 when looking for the max z-index in a display list. r=miko

This restores the previous behaviour (where a result < 0 was ignored), since using a negative value means we place our overlay scrollbar/resizer behind non-positioned content.

Differential Revision: https://phabricator.services.mozilla.com/D20212

--HG--
extra : moz-landing-system : lando

layout/generic/nsGfxScrollFrame.cpp

@@ -2939,10 +2939,14 @@ static Maybe<int32_t> MaxZIndexInList(nsDisplayList* aList,
   Maybe<int32_t> maxZIndex = Nothing();
   for (nsDisplayItem* item = aList->GetBottom(); item;
        item = item->GetAbove()) {
+    int32_t zIndex = item->ZIndex();
+    if (zIndex < 0) {
+      continue;
+    }
     if (!maxZIndex) {
-      maxZIndex = Some(item->ZIndex());
+      maxZIndex = Some(zIndex);
     } else {
-      maxZIndex = Some(std::max(maxZIndex.value(), item->ZIndex()));
+      maxZIndex = Some(std::max(maxZIndex.value(), zIndex));
     }
   }
   return maxZIndex;