Record viewport unit usage and generate proper restyle hint.

components/style/gecko/generated/bindings.rs

@@ -1921,8 +1921,9 @@ extern "C" {
     pub fn Servo_StyleSet_RebuildData(set: RawServoStyleSetBorrowed);
 }
 extern "C" {
-    pub fn Servo_StyleSet_MediumFeaturesChanged(set: RawServoStyleSetBorrowed)
+    pub fn Servo_StyleSet_MediumFeaturesChanged(set: RawServoStyleSetBorrowed,
-     -> bool;
+                                                viewport_changed: bool)
+     -> nsRestyleHint;
 }
 extern "C" {
     pub fn Servo_StyleSet_CompatModeChanged(raw_data:

components/style/gecko/media_queries.rs

@@ -54,6 +54,9 @@ pub struct Device {
     /// Whether any styles computed in the document relied on the root font-size
     /// by using rem units.
     used_root_font_size: AtomicBool,
+    /// Whether any styles computed in the document relied on the viewport size
+    /// by using vw/vh/vmin/vmax units.
+    used_viewport_size: AtomicBool,
 }
 
 unsafe impl Sync for Device {}
@@ -69,6 +72,7 @@ impl Device {
             viewport_override: None,
             root_font_size: AtomicIsize::new(font_size::get_initial_value().0 as isize), // FIXME(bz): Seems dubious?
             used_root_font_size: AtomicBool::new(false),
+            used_viewport_size: AtomicBool::new(false),
         }
     }
 
@@ -112,6 +116,7 @@ impl Device {
         self.viewport_override = None;
         self.default_values = ComputedValues::default_values(self.pres_context());
         self.used_root_font_size.store(false, Ordering::Relaxed);
+        self.used_viewport_size.store(false, Ordering::Relaxed);
     }
 
     /// Returns whether we ever looked up the root font size of the Device.
@@ -146,6 +151,7 @@ impl Device {
 
     /// Returns the current viewport size in app units.
     pub fn au_viewport_size(&self) -> Size2D<Au> {
+        self.used_viewport_size.store(true, Ordering::Relaxed);
         self.viewport_override.as_ref().map(|v| {
             Size2D::new(Au::from_f32_px(v.size.width),
                         Au::from_f32_px(v.size.height))
@@ -156,6 +162,11 @@ impl Device {
         })
     }
 
+    /// Returns whether we ever looked up the viewport size of the Device.
+    pub fn used_viewport_size(&self) -> bool {
+        self.used_viewport_size.load(Ordering::Relaxed)
+    }
+
     /// Returns the device pixel ratio.
     pub fn device_pixel_ratio(&self) -> ScaleFactor<f32, CSSPixel, DevicePixel> {
         let override_dppx = self.pres_context().mOverrideDPPX;

components/style/stylesheets/viewport_rule.rs

@@ -696,6 +696,10 @@ impl MaybeNew for ViewportConstraints {
         //
         // Note: DEVICE-ADAPT § 5. states that relative length values are
         // resolved against initial values
+        //
+        // Note, we set used_viewport_size flag for Gecko in au_viewport_size.
+        // If we ever start supporting ViewportRule in Gecko, we probably want
+        // to avoid doing so at this place.
         let initial_viewport = device.au_viewport_size();
 
         let provider = get_metrics_provider_for_product();

ports/geckolib/glue.rs

@@ -865,7 +865,8 @@ pub extern "C" fn Servo_StyleSet_AppendStyleSheet(
 #[no_mangle]
 pub extern "C" fn Servo_StyleSet_MediumFeaturesChanged(
     raw_data: RawServoStyleSetBorrowed,
-) -> bool {
+    viewport_changed: bool,
+) -> nsRestyleHint {
     let global_style_data = &*GLOBAL_STYLE_DATA;
     let guard = global_style_data.shared_lock.read();
 
@@ -881,11 +882,19 @@ pub extern "C" fn Servo_StyleSet_MediumFeaturesChanged(
     // less often.
     let mut data = PerDocumentStyleData::from_ffi(raw_data).borrow_mut();
 
+    let viewport_units_used = data.stylist.device().used_viewport_size();
     data.stylist.device_mut().reset_computed_values();
-    data.stylist.media_features_change_changed_style(
+    let rules_changed = data.stylist.media_features_change_changed_style(
         data.stylesheets.iter(),
         &guard,
-    )
+    );
+    if rules_changed {
+        structs::nsRestyleHint_eRestyle_Subtree
+    } else if viewport_changed && viewport_units_used {
+        structs::nsRestyleHint_eRestyle_ForceDescendants
+    } else {
+        nsRestyleHint(0)
+    }
 }
 
 #[no_mangle]