Let Stylist compute and store viewport constraints when setting the device

components/layout/layout_task.rs

@@ -1077,7 +1077,7 @@ impl LayoutTask {
                     flow_ref::deref_mut(layout_root));
                 let root_size = {
                     let root_flow = flow::base(&**layout_root);
-                    if rw_data.stylist.constrain_viewport().is_some() {
+                    if rw_data.stylist.get_viewport_constraints().is_some() {
                         root_flow.position.size.to_physical(root_flow.writing_mode)
                     } else {
                         root_flow.overflow.size
@@ -1164,7 +1164,7 @@ impl LayoutTask {
         let device = Device::new(MediaType::Screen, initial_viewport);
         rw_data.stylist.set_device(device);
 
-        let constraints = rw_data.stylist.constrain_viewport();
+        let constraints = rw_data.stylist.get_viewport_constraints();
         rw_data.viewport_size = match constraints {
             Some(ref constraints) => {
                 debug!("Viewport constraints: {:?}", constraints);

components/style/selector_matching.rs

@@ -3,7 +3,7 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 use legacy::PresentationalHintSynthesis;
-use media_queries::Device;
+use media_queries::{Device, MediaType};
 use node::TElementAttributes;
 use properties::{PropertyDeclaration, PropertyDeclarationBlock};
 use restyle_hints::{RestyleHint, StateDependencySet};
@@ -63,6 +63,9 @@ pub struct Stylist {
     // Device that the stylist is currently evaluating against.
     pub device: Device,
 
+    // Viewport constraints based on the current device.
+    viewport_constraints: Option<ViewportConstraints>,
+
     // If true, a stylesheet has been added or the device has
     // changed, and the stylist needs to be updated.
     is_dirty: bool,
@@ -83,6 +86,7 @@ impl Stylist {
     pub fn new(device: Device) -> Stylist {
         let stylist = Stylist {
             stylesheets: vec!(),
+            viewport_constraints: None,
             device: device,
             is_dirty: true,
 
@@ -101,14 +105,6 @@ impl Stylist {
         &self.stylesheets
     }
 
-    pub fn constrain_viewport(&self) -> Option<ViewportConstraints> {
-        let cascaded_rule = self.stylesheets.iter()
-            .flat_map(|s| s.effective_rules(&self.device).viewport())
-            .cascade();
-
-        ViewportConstraints::maybe_new(self.device.viewport_size, &cascaded_rule)
-    }
-
     pub fn update(&mut self) -> bool {
         if self.is_dirty {
             self.element_map = PerPseudoElementSelectorMap::new();
@@ -187,7 +183,15 @@ impl Stylist {
         self.state_deps.compute_hint(element, current_state, state_change)
     }
 
-    pub fn set_device(&mut self, device: Device) {
+    pub fn set_device(&mut self, mut device: Device) {
+        let cascaded_rule = self.stylesheets.iter()
+            .flat_map(|s| s.effective_rules(&self.device).viewport())
+            .cascade();
+
+        self.viewport_constraints = ViewportConstraints::maybe_new(self.device.viewport_size, &cascaded_rule);
+        if let Some(ref constraints) = self.viewport_constraints {
+            device = Device::new(MediaType::Screen, constraints.size);
+        }
         let is_dirty = self.is_dirty || self.stylesheets.iter()
             .flat_map(|stylesheet| stylesheet.rules().media())
             .any(|media_rule| media_rule.evaluate(&self.device) != media_rule.evaluate(&device));
@@ -196,6 +200,13 @@ impl Stylist {
         self.is_dirty |= is_dirty;
     }
 
+    pub fn get_viewport_constraints(&self) -> Option<ViewportConstraints> {
+        match self.viewport_constraints {
+            Some(ref constraints) => Some(constraints.clone()),
+            None => None
+        }
+    }
+
     pub fn add_quirks_mode_stylesheet(&mut self) {
         match read_resource_file(&["quirks-mode.css"]) {
             Ok(res) => {

components/style_traits/viewport.rs

@@ -20,7 +20,7 @@ define_css_keyword_enum!(Orientation:
                          "landscape" => Landscape);
 
 
-#[derive(Debug, PartialEq, Deserialize, Serialize)]
+#[derive(Clone, Debug, PartialEq, Deserialize, Serialize)]
 pub struct ViewportConstraints {
     pub size: TypedSize2D<ViewportPx, f32>,