style: More fine-grained viewport units invalidation

This should speed up window resize significantly.

Differential Revision: https://phabricator.services.mozilla.com/D118878

components/style/invalidation/element/restyle_hints.rs

@@ -60,7 +60,12 @@ impl RestyleHint {
     /// Returns whether this hint invalidates the element and all its
     /// descendants.
     pub fn contains_subtree(&self) -> bool {
-        self.contains(RestyleHint::RESTYLE_SELF | RestyleHint::RESTYLE_DESCENDANTS)
+        self.contains(Self::restyle_subtree())
+    }
+
+    /// Returns whether we'll recascade all of the descendants.
+    pub fn will_recascade_subtree(&self) -> bool {
+        self.contains_subtree() || self.contains(Self::recascade_subtree())
     }
 
     /// Returns whether we need to restyle this element.

components/style/invalidation/mod.rs

@@ -7,3 +7,4 @@
 pub mod element;
 pub mod media_queries;
 pub mod stylesheets;
+pub mod viewport_units;

components/style/invalidation/viewport_units.rs

@@ -0,0 +1,57 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+//! Invalidates style of all elements that depend on viewport units.
+
+use crate::dom::{TElement, TNode};
+use crate::invalidation::element::restyle_hints::RestyleHint;
+
+/// Invalidates style of all elements that depend on viewport units.
+///
+/// Returns whether any element was invalidated.
+pub fn invalidate<E>(root: E) -> bool
+where
+    E: TElement,
+{
+    debug!("invalidation::viewport_units::invalidate({:?})", root);
+    invalidate_recursively(root)
+}
+
+fn invalidate_recursively<E>(element: E) -> bool
+where
+    E: TElement,
+{
+    let mut data = match element.mutate_data() {
+        Some(data) => data,
+        None => return false,
+    };
+
+    if data.hint.will_recascade_subtree() {
+        debug!("invalidate_recursively: {:?} was already invalid", element);
+        return false;
+    }
+
+    let uses_viewport_units = data.styles.uses_viewport_units();
+    if uses_viewport_units {
+        debug!("invalidate_recursively: {:?} uses viewport units", element);
+        data.hint.insert(RestyleHint::RECASCADE_SELF);
+    }
+
+    let mut any_children_invalid = false;
+    for child in element.traversal_children() {
+        if let Some(child) = child.as_element() {
+            any_children_invalid |= invalidate_recursively(child);
+        }
+    }
+
+    if any_children_invalid {
+        debug!(
+            "invalidate_recursively: Children of {:?} changed, setting dirty descendants",
+            element
+        );
+        unsafe { element.set_dirty_descendants() }
+    }
+
+    uses_viewport_units || any_children_invalid
+}