style: Make the document blocked by the topmost element in the top layer.

Spec: https://html.spec.whatwg.org/multipage/#blocked-by-a-modal-dialog

Depends on D86392

Differential Revision: https://phabricator.services.mozilla.com/D86227

components/style/style_adjuster.rs

@@ -163,6 +163,9 @@ impl<'a, 'b: 'a> StyleAdjuster<'a, 'b> {
     ///        pointer-events: none;
     ///        cursor: default;
     ///
+    /// NOTE: dialog:-moz-topmost-modal-dialog is used to override above
+    /// rules to remove the inertness for the topmost modal dialog.
+    ///
     /// NOTE: If this or the pointer-events tweak is removed, then
     /// minimal-xul.css and the scrollbar style caching need to be tweaked.
     fn adjust_for_inert(&mut self) {