Z-index should be ignored for non-positioned stacking contexts

When a stacking-context is not positioned, its z-index should be
ignored. This is per CSS 2 9.9.1. The only exception to this is when
the z-index is applied to an element with display: flex | inline-flex.
inline-flex does not appear to be implemented at this time so we only
do this for flex.

components/layout/display_list_builder.rs

@@ -1331,7 +1331,7 @@ impl FragmentDisplayListBuilding for Fragment {
         Arc::new(StackingContext::new(display_list,
                                       &border_box,
                                       &overflow,
-                                      self.style().get_box().z_index.number_or_zero(),
+                                      self.effective_z_index(),
                                       filters,
                                       self.style().get_effects().mix_blend_mode,
                                       transform,