Make Stylesheet.dirty_on_viewport_size_change an AtomicBool

components/layout_thread/lib.rs

@@ -1055,7 +1055,7 @@ impl LayoutThread {
                        .send(ConstellationMsg::ViewportConstrained(self.id, constraints))
                        .unwrap();
             }
-            if data.document_stylesheets.iter().any(|sheet| sheet.dirty_on_viewport_size_change) {
+            if data.document_stylesheets.iter().any(|sheet| sheet.dirty_on_viewport_size_change()) {
                 let mut iter = node.traverse_preorder();
 
                 let mut next = iter.next();

components/script/dom/htmlmetaelement.rs

@@ -21,6 +21,7 @@ use html5ever_atoms::LocalName;
 use parking_lot::RwLock;
 use std::ascii::AsciiExt;
 use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
 use style::attr::AttrValue;
 use style::str::HTML_SPACE_CHARACTERS;
 use style::stylesheets::{Stylesheet, CssRule, Origin};
@@ -101,7 +102,7 @@ impl HTMLMetaElement {
                         media: Default::default(),
                         // Viewport constraints are always recomputed on resize; they don't need to
                         // force all styles to be recomputed.
-                        dirty_on_viewport_size_change: false,
+                        dirty_on_viewport_size_change: AtomicBool::new(false),
                     }));
                     let doc = document_from_node(self);
                     doc.invalidate_stylesheets();

components/style/stylesheets.rs

@@ -22,6 +22,7 @@ use servo_url::ServoUrl;
 use std::cell::Cell;
 use std::fmt;
 use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, Ordering};
 use style_traits::ToCss;
 use viewport::ViewportRule;
 
@@ -59,7 +60,7 @@ pub struct Stylesheet {
     /// List of media associated with the Stylesheet.
     pub media: Arc<RwLock<MediaList>>,
     pub origin: Origin,
-    pub dirty_on_viewport_size_change: bool,
+    pub dirty_on_viewport_size_change: AtomicBool,
 }
 
 
@@ -215,11 +216,29 @@ impl Stylesheet {
             origin: origin,
             rules: rules.into(),
             media: Arc::new(RwLock::new(media)),
-            dirty_on_viewport_size_change:
+            dirty_on_viewport_size_change: AtomicBool::new(input.seen_viewport_percentages()),
-                input.seen_viewport_percentages(),
         }
     }
 
+    pub fn dirty_on_viewport_size_change(&self) -> bool {
+        self.dirty_on_viewport_size_change.load(Ordering::SeqCst)
+    }
+
+    /// When CSSOM inserts a rule or declaration into this stylesheet, it needs to call this method
+    /// with the return value of `cssparser::Parser::seen_viewport_percentages`.
+    ///
+    /// FIXME: actually make these calls
+    ///
+    /// Note: when *removing* a rule or declaration that contains a viewport percentage,
+    /// to keep the flag accurate we’d need to iterator through the rest of the stylesheet to
+    /// check for *other* such values.
+    ///
+    /// Instead, we conservatively assume there might be some.
+    /// Restyling will some some more work than necessary, but give correct results.
+    pub fn inserted_has_viewport_percentages(&self, has_viewport_percentages: bool) {
+        self.dirty_on_viewport_size_change.fetch_or(has_viewport_percentages, Ordering::SeqCst);
+    }
+
     /// Returns whether the style-sheet applies for the current device depending
     /// on the associated MediaList.
     ///

tests/unit/style/stylesheets.rs

@@ -12,6 +12,7 @@ use servo_url::ServoUrl;
 use std::borrow::ToOwned;
 use std::sync::Arc;
 use std::sync::Mutex;
+use std::sync::atomic::AtomicBool;
 use style::error_reporting::ParseErrorReporter;
 use style::keyframes::{Keyframe, KeyframeSelector, KeyframePercentage};
 use style::parser::ParserContextExtraData;
@@ -55,7 +56,7 @@ fn test_parse_stylesheet() {
     let expected = Stylesheet {
         origin: Origin::UserAgent,
         media: Default::default(),
-        dirty_on_viewport_size_change: false,
+        dirty_on_viewport_size_change: AtomicBool::new(false),
         rules: vec![
             CssRule::Namespace(Arc::new(RwLock::new(NamespaceRule {
                 prefix: None,