Bug 1725721 - Top layer elements that are display:contents should be …

…display: block. r=hiro

As per https://fullscreen.spec.whatwg.org/#new-stacking-layer:

    If its specified display property is contents, it computes to block.

Differential Revision: https://phabricator.services.mozilla.com/D131585

servo/components/style/style_adjuster.rs

@@ -137,9 +137,15 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
     ///    computed to 'absolute' if the element is in a top layer.
     ///
     fn adjust_for_top_layer(&mut self) {
-        if !self.style.is_absolutely_positioned() && self.style.in_top_layer() {
+        if !self.style.in_top_layer() {
+            return;
+        }
+        if !self.style.is_absolutely_positioned() {
             self.style.mutate_box().set_position(Position::Absolute);
         }
+        if self.style.get_box().clone_display().is_contents() {
+            self.style.mutate_box().set_display(Display::Block);
+        }
     }
 
     /// CSS 2.1 section 9.7: