style: Prototype :focus-visible behind a flag.

The heuristic is that we show focus outlines for unknown or key focus, and not
for mouse / touch.

This is probably not the final heuristic we take, but this allows people to play
with it and file bugs.

Once this is mature enough we should remove :-moz-focusring in favor of
:focus-visible.

Differential Revision: https://phabricator.services.mozilla.com/D63861

components/style/element_state.rs

@@ -137,6 +137,10 @@ bitflags! {
         const IN_AUTOFILL_STATE = 1 << 50;
         /// Non-standard & undocumented.
         const IN_AUTOFILL_PREVIEW_STATE = 1 << 51;
+        /// :focus-visible
+        ///
+        /// https://drafts.csswg.org/selectors-4/#the-focus-visible-pseudo
+        const IN_FOCUS_VISIBLE_STATE = 1 << 52;
     }
 }
 

components/style/gecko/non_ts_pseudo_class_list.rs

@@ -42,6 +42,7 @@ macro_rules! apply_non_ts_list {
                 ("enabled", Enabled, enabled, IN_ENABLED_STATE, _),
                 ("focus", Focus, focus, IN_FOCUS_STATE, _),
                 ("focus-within", FocusWithin, focusWithin, IN_FOCUS_WITHIN_STATE, _),
+                ("focus-visible", FocusVisible, focusVisible, IN_FOCUS_VISIBLE_STATE, _),
                 ("hover", Hover, hover, IN_HOVER_STATE, _),
                 ("-moz-drag-over", MozDragOver, mozDragOver, IN_DRAGOVER_STATE, _),
                 ("target", Target, target, IN_TARGET_STATE, _),

components/style/gecko/selector_parser.rs

@@ -172,6 +172,9 @@ impl NonTSPseudoClass {
     /// Returns whether the pseudo-class is enabled in content sheets.
     #[inline]
     fn is_enabled_in_content(&self) -> bool {
+        if matches!(*self, NonTSPseudoClass::FocusVisible) {
+            return static_prefs::pref!("layout.css.focus-visible.enabled")
+        }
         !self.has_any_flag(NonTSPseudoClassFlag::PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME)
     }
 

components/style/gecko/wrapper.rs

@@ -2064,6 +2064,7 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
             NonTSPseudoClass::MozReadOnly |
             NonTSPseudoClass::MozReadWrite |
             NonTSPseudoClass::FocusWithin |
+            NonTSPseudoClass::FocusVisible |
             NonTSPseudoClass::MozDragOver |
             NonTSPseudoClass::MozDevtoolsHighlighted |
             NonTSPseudoClass::MozStyleeditorTransitioning |