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>

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) => {
-                self.handle_theme_change(theme);
+            EmbedderToConstellationMessage::ThemeChange(webview_id, 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(
-            webview_id,
-            WebView {
-                focused_browsing_context_id: browsing_context_id,
-                session_history: JointSessionHistory::new(),
-            },
-        );
+        self.webviews
+            .add(webview_id, ConstellationWebView::new(browsing_context_id));
 
         // 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 {
-                focused_browsing_context_id: new_browsing_context_id,
-                session_history: JointSessionHistory::new(),
-            },
+            ConstellationWebView::new(new_browsing_context_id),
         );
 
         // 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 let Err(err) = pipeline.event_loop.send(msg) {
+            if pipeline.webview_id != webview_id {
+                continue;
+            }
+            if let Err(error) = pipeline
+                .event_loop
+                .send(ScriptThreadMessage::ThemeChange(pipeline.id, theme))
+            {
                 warn!(
-                    "{}: Failed to send theme change event to pipeline ({:?}).",
-                    pipeline.id, err
+                    "{}: Failed to send theme change event to pipeline ({error:?}).",
+                    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,