constellation: Pass system theme to new Pipelines (#37132)

Previously, when the theme was set it was only set on currently active
`Window`s. This change makes setting the `Theme` stateful. Now the
`Constellation` tracks what theme is applied to a `WebView` and properly
passes that value to new `Pipeline`s when they are constructed. In
addition, the value is passed to layout when that is constructed as
well.

Testing: this change adds a unit test.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

Cargo.lock

@@ -1925,6 +1925,7 @@ dependencies = [
  "servo_malloc_size_of",
  "servo_url",
  "strum_macros",
+ "stylo",
  "stylo_traits",
  "url",
  "webdriver",

components/constellation/constellation.rs

@@ -172,6 +172,7 @@ use crate::browsingcontext::{
     AllBrowsingContextsIterator, BrowsingContext, FullyActiveBrowsingContextsIterator,
     NewBrowsingContextInfo,
 };
+use crate::constellation_webview::ConstellationWebView;
 use crate::event_loop::EventLoop;
 use crate::pipeline::{InitialPipelineState, Pipeline};
 use crate::process_manager::ProcessManager;
@@ -229,18 +230,6 @@ struct WebrenderWGPU {
     wgpu_image_map: WGPUImageMap,
 }
 
-/// Servo supports multiple top-level browsing contexts or “webviews”, so `Constellation` needs to
-/// store webview-specific data for bookkeeping.
-struct WebView {
-    /// The currently focused browsing context in this webview for key events.
-    /// The focused pipeline is the current entry of the focused browsing
-    /// context.
-    focused_browsing_context_id: BrowsingContextId,
-
-    /// The joint session history for this webview.
-    session_history: JointSessionHistory,
-}
-
 /// A browsing context group.
 ///
 /// <https://html.spec.whatwg.org/multipage/#browsing-context-group>
@@ -324,7 +313,7 @@ pub struct Constellation<STF, SWF> {
     compositor_proxy: CompositorProxy,
 
     /// Bookkeeping data for all webviews in the constellation.
-    webviews: WebViewManager<WebView>,
+    webviews: WebViewManager<ConstellationWebView>,
 
     /// Channels for the constellation to send messages to the public
     /// resource-related threads. There are two groups of resource threads: one
@@ -895,6 +884,16 @@ where
         if self.shutting_down {
             return;
         }
+
+        let Some(theme) = self
+            .webviews
+            .get(webview_id)
+            .map(ConstellationWebView::theme)
+        else {
+            warn!("Tried to create Pipeline for uknown WebViewId: {webview_id:?}");
+            return;
+        };
+
         debug!(
             "{}: Creating new pipeline in {}",
             pipeline_id, browsing_context_id
@@ -973,6 +972,7 @@ where
             time_profiler_chan: self.time_profiler_chan.clone(),
             mem_profiler_chan: self.mem_profiler_chan.clone(),
             viewport_details: initial_viewport_details,
+            theme,
             event_loop,
             load_data,
             prev_throttled: throttled,
@@ -1436,8 +1436,8 @@ where
                     size_type,
                 );
             },
-            EmbedderToConstellationMessage::ThemeChange(theme) => {
+            EmbedderToConstellationMessage::ThemeChange(webview_id, theme) => {
-                self.handle_theme_change(theme);
+                self.handle_theme_change(webview_id, theme);
             },
             EmbedderToConstellationMessage::TickAnimation(webview_ids) => {
                 self.handle_tick_animation(webview_ids)
@@ -3142,13 +3142,8 @@ where
 
         // Register this new top-level browsing context id as a webview and set
         // its focused browsing context to be itself.
-        self.webviews.add(
+        self.webviews
-            webview_id,
+            .add(webview_id, ConstellationWebView::new(browsing_context_id));
-            WebView {
-                focused_browsing_context_id: browsing_context_id,
-                session_history: JointSessionHistory::new(),
-            },
-        );
 
         // https://html.spec.whatwg.org/multipage/#creating-a-new-browsing-context-group
         let mut new_bc_group: BrowsingContextGroup = Default::default();
@@ -3554,10 +3549,7 @@ where
         self.pipelines.insert(new_pipeline_id, pipeline);
         self.webviews.add(
             new_webview_id,
-            WebView {
+            ConstellationWebView::new(new_browsing_context_id),
-                focused_browsing_context_id: new_browsing_context_id,
-                session_history: JointSessionHistory::new(),
-            },
         );
 
         // https://html.spec.whatwg.org/multipage/#bcg-append
@@ -5623,18 +5615,31 @@ where
         }
     }
 
-    /// Handle theme change events from the embedder and forward them to the script thread
+    /// Handle theme change events from the embedder and forward them to all appropriate `ScriptThread`s.
     #[cfg_attr(
         feature = "tracing",
         tracing::instrument(skip_all, fields(servo_profiling = true), level = "trace")
     )]
-    fn handle_theme_change(&mut self, theme: Theme) {
+    fn handle_theme_change(&mut self, webview_id: WebViewId, theme: Theme) {
+        let Some(webview) = self.webviews.get_mut(webview_id) else {
+            warn!("Received theme change request for uknown WebViewId: {webview_id:?}");
+            return;
+        };
+        if !webview.set_theme(theme) {
+            return;
+        }
+
         for pipeline in self.pipelines.values() {
-            let msg = ScriptThreadMessage::ThemeChange(pipeline.id, theme);
+            if pipeline.webview_id != webview_id {
-            if let Err(err) = pipeline.event_loop.send(msg) {
+                continue;
+            }
+            if let Err(error) = pipeline
+                .event_loop
+                .send(ScriptThreadMessage::ThemeChange(pipeline.id, theme))
+            {
                 warn!(
-                    "{}: Failed to send theme change event to pipeline ({:?}).",
+                    "{}: Failed to send theme change event to pipeline ({error:?}).",
-                    pipeline.id, err
+                    pipeline.id,
                 );
             }
         }

components/constellation/constellation_webview.rs

@@ -0,0 +1,45 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use base::id::BrowsingContextId;
+use embedder_traits::Theme;
+
+use crate::session_history::JointSessionHistory;
+
+/// The `Constellation`'s view of a `WebView` in the embedding layer. This tracks all of the
+/// `Constellation` state for this `WebView`.
+pub(crate) struct ConstellationWebView {
+    /// The currently focused browsing context in this webview for key events.
+    /// The focused pipeline is the current entry of the focused browsing
+    /// context.
+    pub focused_browsing_context_id: BrowsingContextId,
+
+    /// The joint session history for this webview.
+    pub session_history: JointSessionHistory,
+
+    /// The [`Theme`] that this [`ConstellationWebView`] uses. This is communicated to all
+    /// `ScriptThread`s so that they know how to render the contents of a particular `WebView.
+    theme: Theme,
+}
+
+impl ConstellationWebView {
+    pub(crate) fn new(focused_browsing_context_id: BrowsingContextId) -> Self {
+        Self {
+            focused_browsing_context_id,
+            session_history: JointSessionHistory::new(),
+            theme: Theme::Light,
+        }
+    }
+
+    /// Set the [`Theme`] on this [`ConstellationWebView`] returning true if the theme changed.
+    pub(crate) fn set_theme(&mut self, new_theme: Theme) -> bool {
+        let old_theme = std::mem::replace(&mut self.theme, new_theme);
+        old_theme != self.theme
+    }
+
+    /// Get the [`Theme`] of this [`ConstellationWebView`].
+    pub(crate) fn theme(&self) -> Theme {
+        self.theme
+    }
+}

components/constellation/lib.rs

@@ -9,6 +9,7 @@ mod tracing;
 
 mod browsingcontext;
 mod constellation;
+mod constellation_webview;
 mod event_loop;
 mod logging;
 mod pipeline;

components/constellation/pipeline.rs

@@ -25,7 +25,7 @@ use constellation_traits::{LoadData, SWManagerMsg, ScriptToConstellationChan};
 use crossbeam_channel::{Sender, unbounded};
 use devtools_traits::{DevtoolsControlMsg, ScriptToDevtoolsControlMsg};
 use embedder_traits::user_content_manager::UserContentManager;
-use embedder_traits::{AnimationState, FocusSequenceNumber, ViewportDetails};
+use embedder_traits::{AnimationState, FocusSequenceNumber, Theme, ViewportDetails};
 use fonts::{SystemFontServiceProxy, SystemFontServiceProxySender};
 use ipc_channel::Error;
 use ipc_channel::ipc::{self, IpcReceiver, IpcSender};
@@ -61,7 +61,7 @@ pub struct Pipeline {
     /// The ID of the browsing context that contains this Pipeline.
     pub browsing_context_id: BrowsingContextId,
 
-    /// The ID of the top-level browsing context that contains this Pipeline.
+    /// The [`WebViewId`] of the `WebView` that contains this Pipeline.
     pub webview_id: WebViewId,
 
     pub opener: Option<BrowsingContextId>,
@@ -170,6 +170,9 @@ pub struct InitialPipelineState {
     /// The initial [`ViewportDetails`] to use when starting this new [`Pipeline`].
     pub viewport_details: ViewportDetails,
 
+    /// The initial [`Theme`] to use when starting this new [`Pipeline`].
+    pub theme: Theme,
+
     /// The ID of the pipeline namespace for this script thread.
     pub pipeline_namespace_id: PipelineNamespaceId,
 
@@ -224,6 +227,7 @@ impl Pipeline {
                     opener: state.opener,
                     load_data: state.load_data.clone(),
                     viewport_details: state.viewport_details,
+                    theme: state.theme,
                 };
 
                 if let Err(e) = script_chan.send(ScriptThreadMessage::AttachLayout(new_layout_info))
@@ -280,6 +284,7 @@ impl Pipeline {
                     time_profiler_chan: state.time_profiler_chan,
                     mem_profiler_chan: state.mem_profiler_chan,
                     viewport_details: state.viewport_details,
+                    theme: state.theme,
                     script_chan: script_chan.clone(),
                     load_data: state.load_data.clone(),
                     script_port,
@@ -494,6 +499,7 @@ pub struct UnprivilegedPipelineContent {
     time_profiler_chan: time::ProfilerChan,
     mem_profiler_chan: profile_mem::ProfilerChan,
     viewport_details: ViewportDetails,
+    theme: Theme,
     script_chan: IpcSender<ScriptThreadMessage>,
     load_data: LoadData,
     script_port: IpcReceiver<ScriptThreadMessage>,
@@ -544,6 +550,7 @@ impl UnprivilegedPipelineContent {
                 memory_profiler_sender: self.mem_profiler_chan.clone(),
                 devtools_server_sender: self.devtools_ipc_sender,
                 viewport_details: self.viewport_details,
+                theme: self.theme,
                 pipeline_namespace_id: self.pipeline_namespace_id,
                 content_process_shutdown_sender: content_process_shutdown_chan,
                 webgl_chan: self.webgl_chan,

components/layout/layout_impl.rs

@@ -15,7 +15,7 @@ use base::Epoch;
 use base::id::{PipelineId, WebViewId};
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::ScrollState;
-use embedder_traits::{UntrustedNodeAddress, ViewportDetails};
+use embedder_traits::{Theme, UntrustedNodeAddress, ViewportDetails};
 use euclid::default::{Point2D as UntypedPoint2D, Rect as UntypedRect};
 use euclid::{Point2D, Scale, Size2D, Vector2D};
 use fnv::FnvHashMap;
@@ -503,8 +503,7 @@ impl LayoutThread {
             Scale::new(config.viewport_details.hidpi_scale_factor.get()),
             Box::new(LayoutFontMetricsProvider(config.font_context.clone())),
             ComputedValues::initial_values_with_font_override(font),
-            // TODO: obtain preferred color scheme from embedder
+            config.theme.into(),
-            PrefersColorScheme::Light,
         );
 
         LayoutThread {
@@ -951,7 +950,8 @@ impl LayoutThread {
         size_did_change || pixel_ratio_did_change
     }
 
-    fn theme_did_change(&self, theme: PrefersColorScheme) -> bool {
+    fn theme_did_change(&self, theme: Theme) -> bool {
+        let theme: PrefersColorScheme = theme.into();
         theme != self.device().color_scheme()
     }
 
@@ -959,7 +959,7 @@ impl LayoutThread {
     fn update_device(
         &mut self,
         viewport_details: ViewportDetails,
-        theme: PrefersColorScheme,
+        theme: Theme,
         guards: &StylesheetGuards,
     ) {
         let device = Device::new(
@@ -969,7 +969,7 @@ impl LayoutThread {
             Scale::new(viewport_details.hidpi_scale_factor.get()),
             Box::new(LayoutFontMetricsProvider(self.font_context.clone())),
             self.stylist.device().default_computed_values().to_arc(),
-            theme,
+            theme.into(),
         );
 
         // Preserve any previously computed root font size.

components/script/dom/htmliframeelement.rs

@@ -223,6 +223,7 @@ impl HTMLIFrameElement {
                     old_pipeline_id,
                     sandbox: sandboxed,
                     viewport_details,
+                    theme: window.theme(),
                 };
                 window
                     .as_global_scope()
@@ -238,6 +239,7 @@ impl HTMLIFrameElement {
                     opener: None,
                     load_data,
                     viewport_details,
+                    theme: window.theme(),
                 };
 
                 self.pipeline_id.set(Some(new_pipeline_id));
@@ -250,6 +252,7 @@ impl HTMLIFrameElement {
                     old_pipeline_id,
                     sandbox: sandboxed,
                     viewport_details,
+                    theme: window.theme(),
                 };
                 window
                     .as_global_scope()

components/script/dom/window.rs

@@ -80,7 +80,6 @@ use style::dom::OpaqueNode;
 use style::error_reporting::{ContextualParseError, ParseErrorReporter};
 use style::properties::PropertyId;
 use style::properties::style_structs::Font;
-use style::queries::values::PrefersColorScheme;
 use style::selector_parser::PseudoElement;
 use style::str::HTML_SPACE_CHARACTERS;
 use style::stylesheets::UrlExtraData;
@@ -269,7 +268,7 @@ pub(crate) struct Window {
 
     /// Platform theme.
     #[no_trace]
-    theme: Cell<PrefersColorScheme>,
+    theme: Cell<Theme>,
 
     /// Parent id associated with this page, if any.
     #[no_trace]
@@ -2739,13 +2738,13 @@ impl Window {
         self.viewport_details.get()
     }
 
+    /// Get the theme of this [`Window`].
+    pub(crate) fn theme(&self) -> Theme {
+        self.theme.get()
+    }
+
     /// Handle a theme change request, triggering a reflow is any actual change occured.
     pub(crate) fn handle_theme_change(&self, new_theme: Theme) {
-        let new_theme = match new_theme {
-            Theme::Light => PrefersColorScheme::Light,
-            Theme::Dark => PrefersColorScheme::Dark,
-        };
-
         if self.theme.get() == new_theme {
             return;
         }
@@ -3033,6 +3032,7 @@ impl Window {
         player_context: WindowGLContext,
         #[cfg(feature = "webgpu")] gpu_id_hub: Arc<IdentityHub>,
         inherited_secure_context: Option<bool>,
+        theme: Theme,
     ) -> DomRoot<Self> {
         let error_reporter = CSSErrorReporter {
             pipelineid: pipeline_id,
@@ -3118,7 +3118,7 @@ impl Window {
             throttled: Cell::new(false),
             layout_marker: DomRefCell::new(Rc::new(Cell::new(true))),
             current_event: DomRefCell::new(None),
-            theme: Cell::new(PrefersColorScheme::Light),
+            theme: Cell::new(theme),
             trusted_types: Default::default(),
         });
 

components/script/dom/windowproxy.rs

@@ -329,6 +329,9 @@ impl WindowProxy {
             opener: Some(self.browsing_context_id),
             load_data,
             viewport_details: window.viewport_details(),
+            // Use the current `WebView`'s theme initially, but the embedder may
+            // change this later.
+            theme: window.theme(),
         };
         ScriptThread::process_attach_layout(new_layout_info, document.origin().clone());
         // TODO: if noopener is false, copy the sessionStorage storage area of the creator origin.

components/script/navigation.rs

@@ -12,7 +12,7 @@ use base::cross_process_instant::CrossProcessInstant;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use constellation_traits::LoadData;
 use crossbeam_channel::Sender;
-use embedder_traits::ViewportDetails;
+use embedder_traits::{Theme, ViewportDetails};
 use http::header;
 use net_traits::request::{
     CredentialsMode, InsecureRequestsPolicy, RedirectMode, RequestBuilder, RequestMode,
@@ -159,6 +159,9 @@ pub(crate) struct InProgressLoad {
     /// this load.
     #[no_trace]
     pub(crate) url_list: Vec<ServoUrl>,
+    /// The [`Theme`] to use for this page, once it loads.
+    #[no_trace]
+    pub(crate) theme: Theme,
 }
 
 impl InProgressLoad {
@@ -171,6 +174,7 @@ impl InProgressLoad {
         parent_info: Option<PipelineId>,
         opener: Option<BrowsingContextId>,
         viewport_details: ViewportDetails,
+        theme: Theme,
         origin: MutableOrigin,
         load_data: LoadData,
     ) -> InProgressLoad {
@@ -189,6 +193,7 @@ impl InProgressLoad {
             canceller: Default::default(),
             load_data,
             url_list: vec![url],
+            theme,
         }
     }
 

components/script/script_thread.rs

@@ -403,14 +403,20 @@ impl ScriptThreadFactory for ScriptThread {
                 WebViewId::install(state.webview_id);
                 let roots = RootCollection::new();
                 let _stack_roots = ThreadLocalStackRoots::new(&roots);
-                let id = state.id;
-                let browsing_context_id = state.browsing_context_id;
-                let webview_id = state.webview_id;
-                let parent_info = state.parent_info;
-                let opener = state.opener;
                 let memory_profiler_sender = state.memory_profiler_sender.clone();
-                let viewport_details = state.viewport_details;
 
+                let in_progress_load = InProgressLoad::new(
+                    state.id,
+                    state.browsing_context_id,
+                    state.webview_id,
+                    state.parent_info,
+                    state.opener,
+                    state.viewport_details,
+                    state.theme,
+                    MutableOrigin::new(load_data.url.origin()),
+                    load_data,
+                );
+                let reporter_name = format!("script-reporter-{:?}", state.id);
                 let script_thread = ScriptThread::new(state, layout_factory, system_font_service);
 
                 SCRIPT_THREAD_ROOT.with(|root| {
@@ -419,19 +425,8 @@ impl ScriptThreadFactory for ScriptThread {
 
                 let mut failsafe = ScriptMemoryFailsafe::new(&script_thread);
 
-                let origin = MutableOrigin::new(load_data.url.origin());
+                script_thread.pre_page_load(in_progress_load);
-                script_thread.pre_page_load(InProgressLoad::new(
-                    id,
-                    browsing_context_id,
-                    webview_id,
-                    parent_info,
-                    opener,
-                    viewport_details,
-                    origin,
-                    load_data,
-                ));
 
-                let reporter_name = format!("script-reporter-{:?}", id);
                 memory_profiler_sender.run_with_memory_reporting(
                     || {
                         script_thread.start(CanGc::note());
@@ -2435,6 +2430,7 @@ impl ScriptThread {
             opener,
             load_data,
             viewport_details,
+            theme,
         } = new_layout_info;
 
         // Kick off the fetch for the new resource.
@@ -2446,6 +2442,7 @@ impl ScriptThread {
             parent_info,
             opener,
             viewport_details,
+            theme,
             origin,
             load_data,
         );
@@ -3189,6 +3186,7 @@ impl ScriptThread {
             time_profiler_chan: self.senders.time_profiler_sender.clone(),
             compositor_api: self.compositor_api.clone(),
             viewport_details: incomplete.viewport_details,
+            theme: incomplete.theme,
         };
 
         // Create the window and document objects.
@@ -3228,6 +3226,7 @@ impl ScriptThread {
             #[cfg(feature = "webgpu")]
             self.gpu_id_hub.clone(),
             incomplete.load_data.inherited_secure_context,
+            incomplete.theme,
         );
 
         let _realm = enter_realm(&*window);

components/servo/tests/webview.rs

@@ -17,14 +17,21 @@ use std::rc::Rc;
 use anyhow::ensure;
 use common::{ServoTest, run_api_tests};
 use servo::{
-    JSValue, JavaScriptEvaluationError, LoadStatus, WebView, WebViewBuilder, WebViewDelegate,
+    JSValue, JavaScriptEvaluationError, LoadStatus, Theme, WebView, WebViewBuilder, WebViewDelegate,
 };
+use url::Url;
 
 #[derive(Default)]
 struct WebViewDelegateImpl {
     url_changed: Cell<bool>,
 }
 
+impl WebViewDelegateImpl {
+    pub(crate) fn reset(&self) {
+        self.url_changed.set(false);
+    }
+}
+
 impl WebViewDelegate for WebViewDelegateImpl {
     fn notify_url_changed(&self, _webview: servo::WebView, _url: url::Url) {
         self.url_changed.set(true);
@@ -128,10 +135,40 @@ fn test_create_webview_and_immediately_drop_webview_before_shutdown(
     Ok(())
 }
 
+fn test_theme_change(servo_test: &ServoTest) -> Result<(), anyhow::Error> {
+    let delegate = Rc::new(WebViewDelegateImpl::default());
+    let webview = WebViewBuilder::new(servo_test.servo())
+        .delegate(delegate.clone())
+        .url(Url::parse("data:text/html,page one").unwrap())
+        .build();
+
+    let is_dark_theme_script = "window.matchMedia('(prefers-color-scheme: dark)').matches";
+
+    // The default theme is "light".
+    let result = evaluate_javascript(servo_test, webview.clone(), is_dark_theme_script);
+    ensure!(result == Ok(JSValue::Boolean(false)));
+
+    // Changing the theme updates the current page.
+    webview.notify_theme_change(Theme::Dark);
+    let result = evaluate_javascript(servo_test, webview.clone(), is_dark_theme_script);
+    ensure!(result == Ok(JSValue::Boolean(true)));
+
+    delegate.reset();
+    webview.load(Url::parse("data:text/html,page two").unwrap());
+    servo_test.spin(move || Ok(!delegate.url_changed.get()))?;
+
+    // The theme persists after a navigation.
+    let result = evaluate_javascript(servo_test, webview.clone(), is_dark_theme_script);
+    ensure!(result == Ok(JSValue::Boolean(true)));
+
+    Ok(())
+}
+
 fn main() {
     run_api_tests!(
         test_create_webview,
         test_evaluate_javascript_basic,
+        test_theme_change,
         // This test needs to be last, as it tests creating and dropping
         // a WebView right before shutdown.
         test_create_webview_and_immediately_drop_webview_before_shutdown

components/servo/webview.rs

@@ -395,7 +395,10 @@ impl WebView {
     pub fn notify_theme_change(&self, theme: Theme) {
         self.inner()
             .constellation_proxy
-            .send(EmbedderToConstellationMessage::ThemeChange(theme))
+            .send(EmbedderToConstellationMessage::ThemeChange(
+                self.id(),
+                theme,
+            ))
     }
 
     pub fn load(&self, url: Url) {

components/shared/constellation/from_script_message.rs

@@ -16,7 +16,7 @@ use canvas_traits::canvas::{CanvasId, CanvasMsg};
 use devtools_traits::{DevtoolScriptControlMsg, ScriptToDevtoolsControlMsg, WorkerId};
 use embedder_traits::{
     AnimationState, EmbedderMsg, FocusSequenceNumber, JSValue, JavaScriptEvaluationError,
-    JavaScriptEvaluationId, MediaSessionEvent, TouchEventResult, ViewportDetails,
+    JavaScriptEvaluationId, MediaSessionEvent, Theme, TouchEventResult, ViewportDetails,
     WebDriverMessageId,
 };
 use euclid::default::Size2D as UntypedSize2D;
@@ -417,6 +417,8 @@ pub struct IFrameLoadInfoWithData {
     pub sandbox: IFrameSandboxState,
     /// The initial viewport size for this iframe.
     pub viewport_details: ViewportDetails,
+    /// The [`Theme`] to use within this iframe.
+    pub theme: Theme,
 }
 
 /// Resources required by workerglobalscopes

components/shared/constellation/lib.rs

@@ -55,7 +55,7 @@ pub enum EmbedderToConstellationMessage {
     /// Inform the Constellation that a `WebView`'s [`ViewportDetails`] have changed.
     ChangeViewportDetails(WebViewId, ViewportDetails, WindowSizeType),
     /// Inform the constellation of a theme change.
-    ThemeChange(Theme),
+    ThemeChange(WebViewId, Theme),
     /// Requests that the constellation instruct script/layout to try to layout again and tick
     /// animations.
     TickAnimation(Vec<WebViewId>),

components/shared/embedder/Cargo.toml

@@ -35,6 +35,7 @@ serde = { workspace = true }
 servo_url = { path = "../../url" }
 strum_macros = { workspace = true }
 stylo_traits = { workspace = true }
+stylo = { workspace = true }
 url = { workspace = true }
 webdriver = { workspace = true }
 webrender_api = { workspace = true }

components/shared/embedder/lib.rs

@@ -34,6 +34,7 @@ use pixels::Image;
 use serde::{Deserialize, Deserializer, Serialize, Serializer};
 use servo_url::ServoUrl;
 use strum_macros::IntoStaticStr;
+use style::queries::values::PrefersColorScheme;
 use style_traits::CSSPixel;
 use url::Url;
 use webrender_api::units::{DeviceIntPoint, DeviceIntRect, DeviceIntSize, DevicePixel};
@@ -598,6 +599,16 @@ pub enum Theme {
     /// Dark theme.
     Dark,
 }
+
+impl From<Theme> for PrefersColorScheme {
+    fn from(value: Theme) -> Self {
+        match value {
+            Theme::Light => PrefersColorScheme::Light,
+            Theme::Dark => PrefersColorScheme::Dark,
+        }
+    }
+}
+
 // The type of MediaSession action.
 /// <https://w3c.github.io/mediasession/#enumdef-mediasessionaction>
 #[derive(Clone, Debug, Deserialize, Eq, Hash, MallocSizeOf, PartialEq, Serialize)]

components/shared/script/lib.rs

@@ -68,6 +68,8 @@ pub struct NewLayoutInfo {
     pub load_data: LoadData,
     /// Initial [`ViewportDetails`] for this layout.
     pub viewport_details: ViewportDetails,
+    /// The [`Theme`] of the new layout.
+    pub theme: Theme,
 }
 
 /// When a pipeline is closed, should its browsing context be discarded too?
@@ -321,6 +323,8 @@ pub struct InitialScriptState {
     pub devtools_server_sender: Option<IpcSender<ScriptToDevtoolsControlMsg>>,
     /// Initial [`ViewportDetails`] for the frame that is initiating this `ScriptThread`.
     pub viewport_details: ViewportDetails,
+    /// Initial [`Theme`] for the frame that is initiating this `ScriptThread`.
+    pub theme: Theme,
     /// The ID of the pipeline namespace for this script thread.
     pub pipeline_namespace_id: PipelineNamespaceId,
     /// A ping will be sent on this channel once the script thread shuts down.

components/shared/script_layout/lib.rs

@@ -20,7 +20,7 @@ use base::Epoch;
 use base::id::{BrowsingContextId, PipelineId, WebViewId};
 use compositing_traits::CrossProcessCompositorApi;
 use constellation_traits::{LoadData, ScrollState};
-use embedder_traits::{UntrustedNodeAddress, ViewportDetails};
+use embedder_traits::{Theme, UntrustedNodeAddress, ViewportDetails};
 use euclid::default::{Point2D, Rect};
 use fnv::FnvHashMap;
 use fonts::{FontContext, SystemFontServiceProxy};
@@ -46,7 +46,6 @@ use style::invalidation::element::restyle_hints::RestyleHint;
 use style::media_queries::Device;
 use style::properties::PropertyId;
 use style::properties::style_structs::Font;
-use style::queries::values::PrefersColorScheme;
 use style::selector_parser::{PseudoElement, RestyleDamage, Snapshot};
 use style::stylesheets::Stylesheet;
 use webrender_api::ImageKey;
@@ -182,6 +181,7 @@ pub struct LayoutConfig {
     pub time_profiler_chan: time::ProfilerChan,
     pub compositor_api: CrossProcessCompositorApi,
     pub viewport_details: ViewportDetails,
+    pub theme: Theme,
 }
 
 pub trait LayoutFactory: Send + Sync {
@@ -428,7 +428,7 @@ pub struct ReflowRequest {
     /// The set of image animations.
     pub node_to_image_animation_map: FxHashMap<OpaqueNode, ImageAnimationState>,
     /// The theme for the window
-    pub theme: PrefersColorScheme,
+    pub theme: Theme,
     /// The node highlighted by the devtools, if any
     pub highlighted_dom_node: Option<OpaqueNode>,
 }