Ensure the default computed values are up-to-date before evaluating media queries.

Zoom changes can change the meaning of ems, so we can't re-evaluate media
queries with the old values, because otherwise we may miss a restyle.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1357461
MozReview-Commit-ID: HQInvR7RPqR

components/style/gecko/data.rs

@@ -9,7 +9,6 @@ use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
 use dom::TElement;
 use fnv::FnvHashMap;
 use gecko::rules::{CounterStyleRule, FontFaceRule};
-use gecko::wrapper::GeckoElement;
 use gecko_bindings::bindings::RawServoStyleSet;
 use gecko_bindings::structs::RawGeckoPresContextOwned;
 use gecko_bindings::structs::nsIDocument;
@@ -70,15 +69,6 @@ impl PerDocumentStyleData {
 }
 
 impl PerDocumentStyleDataImpl {
-    /// Reset the device state because it may have changed.
-    ///
-    /// Implies also a stylesheet flush.
-    pub fn reset_device(&mut self, guard: &SharedRwLockReadGuard) {
-        self.stylist.device_mut().reset();
-        self.stylesheets.force_dirty();
-        self.flush_stylesheets::<GeckoElement>(guard, None);
-    }
-
     /// Recreate the style data if the stylesheets have changed.
     pub fn flush_stylesheets<E>(&mut self,
                                 guard: &SharedRwLockReadGuard,

components/style/gecko/media_queries.rs

@@ -99,6 +99,13 @@ impl Device {
         self.root_font_size.store(size.0 as isize, Ordering::Relaxed)
     }
 
+    /// Recreates the default computed values.
+    pub fn reset_computed_values(&mut self) {
+        // NB: A following stylesheet flush will populate this if appropriate.
+        self.viewport_override = None;
+        self.default_values = ComputedValues::default_values(unsafe { &*self.pres_context });
+    }
+
     /// Recreates all the temporary state that the `Device` stores.
     ///
     /// This includes the viewport override from `@viewport` rules, and also the
@@ -106,7 +113,7 @@ impl Device {
     pub fn reset(&mut self) {
         // NB: A following stylesheet flush will populate this if appropriate.
         self.viewport_override = None;
-        self.default_values = ComputedValues::default_values(unsafe { &*self.pres_context });
+        self.reset_computed_values();
     }
 
     /// Returns the current media type of the device.

ports/geckolib/glue.rs

@@ -758,7 +758,15 @@ pub extern "C" fn Servo_StyleSet_MediumFeaturesChanged(
     // it's up to date.
     //
     // In case it isn't we would trigger a rebuild + restyle as needed too.
-    let data = PerDocumentStyleData::from_ffi(raw_data).borrow();
+    //
+    // We need to ensure the default computed values are up to date though,
+    // because those can influence the result of media query evaluation.
+    //
+    // FIXME(emilio, bug 1369984): do the computation conditionally, to do it
+    // less often.
+    let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
+
+    data.stylist.device_mut().reset_computed_values();
     data.stylist.media_features_change_changed_style(
         data.stylesheets.iter(),
         &guard,
@@ -1428,7 +1436,9 @@ pub extern "C" fn Servo_StyleSet_RebuildData(raw_data: RawServoStyleSetBorrowed)
     let guard = global_style_data.shared_lock.read();
 
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
-    data.reset_device(&guard);
+    data.stylist.device_mut().reset();
+    data.stylesheets.force_dirty();
+    data.flush_stylesheets::<GeckoElement>(&guard, None);
 }
 
 #[no_mangle]