style: Cache computed styles objects display: none subtrees

This reuses our existing undisplayed style generation, but in a
per-document rather than per-nsComputedDOMStyle object, which means that
we can avoid re-resolving styles of elements in display: none subtrees
much more often.

This brings the test-case in the bug to par with other browsers or
better, and is much simpler than the initial approach I tried back in
the day.

Differential Revision: https://phabricator.services.mozilla.com/D147547

components/style/gecko/data.rs

@@ -129,6 +129,12 @@ impl StylesheetInDocument for GeckoStyleSheet {
 pub struct PerDocumentStyleDataImpl {
     /// Rule processor.
     pub stylist: Stylist,
+
+    /// A cache from element to resolved style.
+    pub undisplayed_style_cache: crate::traversal::UndisplayedStyleCache,
+
+    /// The generation for which our cache is valid.
+    pub undisplayed_style_cache_generation: u64,
 }
 
 /// The data itself is an `AtomicRefCell`, which guarantees the proper semantics
@@ -143,6 +149,8 @@ impl PerDocumentStyleData {
 
         PerDocumentStyleData(AtomicRefCell::new(PerDocumentStyleDataImpl {
             stylist: Stylist::new(device, quirks_mode.into()),
+            undisplayed_style_cache: Default::default(),
+            undisplayed_style_cache_generation: 0,
         }))
     }
 
@@ -177,12 +185,6 @@ impl PerDocumentStyleDataImpl {
         self.stylist.device().default_computed_values_arc()
     }
 
-    /// Returns whether visited styles are enabled.
-    #[inline]
-    pub fn visited_styles_enabled(&self) -> bool {
-        unsafe { bindings::Gecko_VisitedStylesEnabled(self.stylist.device().document()) }
-    }
-
     /// Measure heap usage.
     pub fn add_size_of(&self, ops: &mut MallocSizeOfOps, sizes: &mut ServoStyleSetSizes) {
         self.stylist.add_size_of(ops, sizes);

components/style/gecko/media_queries.rs

@@ -411,6 +411,11 @@ impl Device {
         self.used_font_metrics.load(Ordering::Relaxed)
     }
 
+    /// Returns whether visited styles are enabled.
+    pub fn visited_styles_enabled(&self) -> bool {
+        unsafe { bindings::Gecko_VisitedStylesEnabled(self.document()) }
+    }
+
     /// Returns the device pixel ratio.
     pub fn device_pixel_ratio(&self) -> Scale<f32, CSSPixel, DevicePixel> {
         let pc = match self.pres_context() {