layout: More conservatively replace Stylist's Device (#31857)

Instead of replacing Stylist's device on every reflow, only replace it
when the viewport changes. In addition, preserve the root font size from
the previous reflow fixing an issue where `rem` units were not properly
computed between reflows.

This fixes a bug where fonts that are sized using `rem` units change
size on reload.

components/layout_thread/lib.rs

@@ -379,7 +379,10 @@ impl LayoutThread {
             root_flow: RefCell::new(None),
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
-            viewport_size: Size2D::new(Au(0), Au(0)),
+            viewport_size: Size2D::new(
+                Au::from_f32_px(window_size.initial_viewport.width),
+                Au::from_f32_px(window_size.initial_viewport.height),
+            ),
             webrender_api,
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
             rw_data: Arc::new(Mutex::new(LayoutThreadData {
@@ -936,16 +939,6 @@ impl LayoutThread {
         );
         trace!("{:?}", ShowSubtree(root_element.as_node()));
 
-        let initial_viewport = data.window_size.initial_viewport;
-        let device_pixel_ratio = data.window_size.device_pixel_ratio;
-        let old_viewport_size = self.viewport_size;
-        let current_screen_size = Size2D::new(
-            Au::from_f32_px(initial_viewport.width),
-            Au::from_f32_px(initial_viewport.height),
-        );
-
-        let origin = data.origin.clone();
-
         // Calculate the actual viewport as per DEVICE-ADAPT § 6
         // If the entire flow tree is invalid, then it will be reflowed anyhow.
         let document_shared_lock = document.style_shared_lock();
@@ -959,19 +952,7 @@ impl LayoutThread {
         };
 
         let had_used_viewport_units = self.stylist.device().used_viewport_units();
-        let device = Device::new(
+        let viewport_size_changed = self.handle_viewport_change(data.window_size, &guards);
-            MediaType::screen(),
-            self.stylist.quirks_mode(),
-            initial_viewport,
-            device_pixel_ratio,
-        );
-        let sheet_origins_affected_by_device_change = self.stylist.set_device(device, &guards);
-
-        self.stylist
-            .force_stylesheet_origins_dirty(sheet_origins_affected_by_device_change);
-        self.viewport_size = current_screen_size;
-
-        let viewport_size_changed = self.viewport_size != old_viewport_size;
         if viewport_size_changed && had_used_viewport_units {
             if let Some(mut data) = root_element.mutate_data() {
                 data.hint.insert(RestyleHint::recascade_subtree());
@@ -1063,7 +1044,7 @@ impl LayoutThread {
         let mut layout_context = self.build_layout_context(
             guards.clone(),
             &map,
-            origin,
+            data.origin.clone(),
             data.animation_timeline_value,
             &data.animations,
             data.stylesheets_changed,
@@ -1518,6 +1499,42 @@ impl LayoutThread {
             },
         })
     }
+
+    /// Update layout given a new viewport. Returns true if the viewport changed or false if it didn't.
+    fn handle_viewport_change(
+        &mut self,
+        window_size_data: WindowSizeData,
+        guards: &StylesheetGuards,
+    ) -> bool {
+        // If the viewport size and device pixel ratio has not changed, do not make any changes.
+        let au_viewport_size = Size2D::new(
+            Au::from_f32_px(window_size_data.initial_viewport.width),
+            Au::from_f32_px(window_size_data.initial_viewport.height),
+        );
+
+        if self.stylist.device().au_viewport_size() == au_viewport_size &&
+            self.stylist.device().device_pixel_ratio() == window_size_data.device_pixel_ratio
+        {
+            return false;
+        }
+
+        let device = Device::new(
+            MediaType::screen(),
+            self.stylist.quirks_mode(),
+            window_size_data.initial_viewport,
+            window_size_data.device_pixel_ratio,
+        );
+
+        // Preserve any previously computed root font size.
+        device.set_root_font_size(self.stylist.device().root_font_size());
+
+        let sheet_origins_affected_by_device_change = self.stylist.set_device(device, guards);
+        self.stylist
+            .force_stylesheet_origins_dirty(sheet_origins_affected_by_device_change);
+
+        self.viewport_size = au_viewport_size;
+        true
+    }
 }
 
 impl ProfilerMetadataFactory for LayoutThread {

components/layout_thread_2020/lib.rs

@@ -72,6 +72,7 @@ use style::dom::{TElement, TNode};
 use style::driver;
 use style::error_reporting::RustLogReporter;
 use style::global_style_data::{GLOBAL_STYLE_DATA, STYLE_THREAD_POOL};
+use style::invalidation::element::restyle_hints::RestyleHint;
 use style::media_queries::{Device, MediaList, MediaType};
 use style::properties::PropertyId;
 use style::selector_parser::SnapshotMap;
@@ -360,7 +361,10 @@ impl LayoutThread {
             fragment_tree: Default::default(),
             // Epoch starts at 1 because of the initial display list for epoch 0 that we send to WR
             epoch: Cell::new(Epoch(1)),
-            viewport_size: Size2D::new(Au(0), Au(0)),
+            viewport_size: Size2D::new(
+                Au::from_f32_px(window_size.initial_viewport.width),
+                Au::from_f32_px(window_size.initial_viewport.height),
+            ),
             webrender_api: webrender_api_sender,
             stylist: Stylist::new(device, QuirksMode::NoQuirks),
             rw_data: Arc::new(Mutex::new(LayoutThreadData {
@@ -628,15 +632,6 @@ impl LayoutThread {
             Some(x) => x,
         };
 
-        let initial_viewport = data.window_size.initial_viewport;
-        let device_pixel_ratio = data.window_size.device_pixel_ratio;
-        let current_screen_size = Size2D::new(
-            Au::from_f32_px(initial_viewport.width),
-            Au::from_f32_px(initial_viewport.height),
-        );
-
-        let origin = data.origin.clone();
-
         // Calculate the actual viewport as per DEVICE-ADAPT § 6
         // If the entire flow tree is invalid, then it will be reflowed anyhow.
         let document_shared_lock = document.style_shared_lock();
@@ -649,17 +644,14 @@ impl LayoutThread {
             ua_or_user: &ua_or_user_guard,
         };
 
-        let device = Device::new(
+        let had_used_viewport_units = self.stylist.device().used_viewport_units();
-            MediaType::screen(),
+        let viewport_size_changed = self.handle_viewport_change(data.window_size, &guards);
-            self.stylist.quirks_mode(),
+        if viewport_size_changed && had_used_viewport_units {
-            initial_viewport,
+            if let Some(mut data) = root_element.mutate_data() {
-            device_pixel_ratio,
+                data.hint.insert(RestyleHint::recascade_subtree());
-        );
+            }
-        let sheet_origins_affected_by_device_change = self.stylist.set_device(device, &guards);
+        }
 
-        self.stylist
-            .force_stylesheet_origins_dirty(sheet_origins_affected_by_device_change);
-        self.viewport_size = current_screen_size;
         if self.first_reflow.get() {
             for stylesheet in &ua_stylesheets.user_or_user_agent_stylesheets {
                 self.stylist
@@ -739,7 +731,7 @@ impl LayoutThread {
         let mut layout_context = self.build_layout_context(
             guards.clone(),
             &map,
-            origin,
+            data.origin.clone(),
             data.animation_timeline_value,
             &data.animations,
             data.stylesheets_changed,
@@ -1158,6 +1150,42 @@ impl LayoutThread {
             }
         }
     }
+
+    /// Update layout given a new viewport. Returns true if the viewport changed or false if it didn't.
+    fn handle_viewport_change(
+        &mut self,
+        window_size_data: WindowSizeData,
+        guards: &StylesheetGuards,
+    ) -> bool {
+        // If the viewport size and device pixel ratio has not changed, do not make any changes.
+        let au_viewport_size = Size2D::new(
+            Au::from_f32_px(window_size_data.initial_viewport.width),
+            Au::from_f32_px(window_size_data.initial_viewport.height),
+        );
+
+        if self.stylist.device().au_viewport_size() == au_viewport_size &&
+            self.stylist.device().device_pixel_ratio() == window_size_data.device_pixel_ratio
+        {
+            return false;
+        }
+
+        let device = Device::new(
+            MediaType::screen(),
+            self.stylist.quirks_mode(),
+            window_size_data.initial_viewport,
+            window_size_data.device_pixel_ratio,
+        );
+
+        // Preserve any previously computed root font size.
+        device.set_root_font_size(self.stylist.device().root_font_size());
+
+        let sheet_origins_affected_by_device_change = self.stylist.set_device(device, guards);
+        self.stylist
+            .force_stylesheet_origins_dirty(sheet_origins_affected_by_device_change);
+
+        self.viewport_size = au_viewport_size;
+        true
+    }
 }
 
 impl ProfilerMetadataFactory for LayoutThread {

components/script/dom/htmlfontelement.rs

@@ -81,11 +81,13 @@ impl VirtualMethods for HTMLFontElement {
     }
 
     fn attribute_affects_presentational_hints(&self, attr: &Attr) -> bool {
-        if attr.local_name() == &local_name!("color") {
+        if attr.local_name() == &local_name!("color") ||
+            attr.local_name() == &local_name!("size") ||
+            attr.local_name() == &local_name!("face")
+        {
             return true;
         }
 
-        // FIXME: Should also return true for `size` and `face` changes!
         self.super_type()
             .unwrap()
             .attribute_affects_presentational_hints(attr)

tests/wpt/meta-legacy-layout/css/css-values/getComputedStyle-calc-mixed-units-002.html.ini

@@ -1,6 +1,3 @@
 [getComputedStyle-calc-mixed-units-002.html]
-  [testing width: calc(5% + 4rem)]
-    expected: FAIL
-
   [testing width: calc(8lh + 7px)]
     expected: FAIL

tests/wpt/meta-legacy-layout/css/css-values/rem-root-font-size-restyle-1.html.ini

@@ -1,2 +0,0 @@
-[rem-root-font-size-restyle-1.html]
-  expected: FAIL

tests/wpt/meta/css/css-values/getComputedStyle-calc-mixed-units-002.html.ini

@@ -1,6 +1,3 @@
 [getComputedStyle-calc-mixed-units-002.html]
-  [testing width: calc(5% + 4rem)]
-    expected: FAIL
-
   [testing width: calc(8lh + 7px)]
     expected: FAIL

tests/wpt/meta/css/css-values/rem-root-font-size-restyle-1.html.ini

@@ -1,2 +0,0 @@
-[rem-root-font-size-restyle-1.html]
-  expected: FAIL

tests/wpt/mozilla/meta-legacy-layout/mozilla/detached_layout.html.ini

@@ -1,3 +0,0 @@
-[detached_layout.html]
-  [Detached layout doesn't panic]
-    expected: FAIL