Bug 1394226 - Correct z-ordering for some table parts. r=dbaron

MozReview-Commit-ID: IdLhMGDmQag

layout/tables/nsTableFrame.cpp

@@ -1605,6 +1605,8 @@ nsTableFrame::DisplayGenericTablePart(nsDisplayListBuilder* aBuilder,
     }
   }
 
+  aFrame->DisplayOutline(aBuilder, aLists);
+
   aTraversal(aBuilder, aFrame, aLists);
 
   if (isVisible) {
@@ -1625,8 +1627,6 @@ nsTableFrame::DisplayGenericTablePart(nsDisplayListBuilder* aBuilder,
       }
     }
   }
-
-  aFrame->DisplayOutline(aBuilder, aLists);
 }
 
 // table paint code is concerned primarily with borders and bg color

layout/tables/nsTableWrapperFrame.cpp

@@ -187,8 +187,11 @@ nsTableWrapperFrame::BuildDisplayList(nsDisplayListBuilder*   aBuilder,
   BuildDisplayListForChild(aBuilder, mCaptionFrames.FirstChild(), captionSet);
 
   // Now we have to sort everything by content order, since the caption
-  // may be somewhere inside the table
-  set.BlockBorderBackgrounds()->SortByContentOrder(GetContent());
+  // may be somewhere inside the table.
+  // We don't sort BlockBorderBackgrounds and BorderBackgrounds because the
+  // display items in those lists should stay out of content order in order to
+  // follow the rules in https://www.w3.org/TR/CSS21/zindex.html#painting-order
+  // and paint the caption background after all of the rest.
   set.Floats()->SortByContentOrder(GetContent());
   set.Content()->SortByContentOrder(GetContent());
   set.PositionedDescendants()->SortByContentOrder(GetContent());