servoshell: Activate accessibility in all webviews (#43558)

this patch plumbs the webview accessibility trees (#43029, #43556) into
servoshell. we add a global flag in servoshell, which is set when the
platform activates accessibility and cleared when the platform
deactivates accessibility. the flag in turn [activates
accessibility](https://doc.servo.org/servo/struct.WebView.html#method.set_accessibility_active)
in existing and new webviews.

Testing: none in this patch, but will be covered by end-to-end platform
a11y tests in WPT
Fixes: part of #4344, extracted from our work in #42338

Signed-off-by: delan azabani <dazabani@igalia.com>
Co-authored-by: Luke Warlow <lwarlow@igalia.com>
Co-authored-by: Alice Boxhall <alice@igalia.com>

ports/servoshell/desktop/app.rs

@@ -214,7 +214,7 @@ impl ApplicationHandler<AppEvent> for App {
             .and_then(|window_id| state.window(ServoShellWindowId::from(u64::from(window_id))))
         {
             if let Some(headed_window) = window.platform_window().as_headed_window() {
-                headed_window.handle_winit_app_event(&window, app_event);
+                headed_window.handle_winit_app_event(state.clone(), app_event);
             }
         }
 

ports/servoshell/desktop/gui.rs

@@ -59,6 +59,10 @@ pub struct Gui {
     ///
     /// These need to be cached across egui draw calls.
     favicon_textures: HashMap<WebViewId, (egui::TextureHandle, egui::load::SizedTexture)>,
+
+    /// AccessKit tree updates pending the next egui tick.
+    /// This allows us to ensure that graft nodes are sent before the subtrees they graft.
+    pending_accesskit_updates: Vec<accesskit::TreeUpdate>,
 }
 
 fn truncate_with_ellipsis(input: &str, max_length: usize) -> String {
@@ -124,6 +128,7 @@ impl Gui {
             can_go_back: false,
             can_go_forward: false,
             favicon_textures: Default::default(),
+            pending_accesskit_updates: vec![],
         }
     }
 
@@ -472,6 +477,16 @@ impl Gui {
             // If the top parts of the GUI changed size, then update the size of the WebView and also
             // the size of its RenderingContext.
             let rect = ctx.available_rect();
+
+            // Build a graft node for each WebView.
+            for (webview_id, webview) in window.webviews() {
+                if let Some(tree_id) = webview.accesskit_tree_id() {
+                    let id = egui::Id::new(webview_id);
+                    ctx.accesskit_node_builder(id, |node| {
+                        node.set_tree_id(tree_id);
+                    });
+                }
+            }
             let size = Size2D::new(rect.width(), rect.height()) * scale;
             if let Some(webview) = window.active_webview() &&
                 size != webview.size()
@@ -509,6 +524,16 @@ impl Gui {
                 });
             }
         });
+
+        let adapter = self
+            .context
+            .egui_winit
+            .accesskit
+            .as_mut()
+            .expect("guaranteed by Gui::new()");
+        for tree_update in self.pending_accesskit_updates.drain(..) {
+            adapter.update_if_active(|| tree_update);
+        }
     }
 
     /// Paint the GUI, as of the last update.
@@ -618,8 +643,8 @@ impl Gui {
         self.context.egui_ctx.set_zoom_factor(factor);
     }
 
-    pub(crate) fn notify_accessibility_tree_update(&mut self, _tree_update: accesskit::TreeUpdate) {
-        // TODO(#41930): Forward this update to `self.context.egui_winit.accesskit`
+    pub(crate) fn notify_accessibility_tree_update(&mut self, tree_update: accesskit::TreeUpdate) {
+        self.pending_accesskit_updates.push(tree_update);
     }
 }
 

ports/servoshell/desktop/headed_window.rs

@@ -761,9 +761,21 @@ impl HeadedWindow {
         }
     }
 
-    pub(crate) fn handle_winit_app_event(&self, _window: &ServoShellWindow, app_event: AppEvent) {
+    pub(crate) fn handle_winit_app_event(&self, state: Rc<RunningAppState>, app_event: AppEvent) {
         if let AppEvent::Accessibility(ref event) = app_event {
-            // TODO(#41930): Forward accesskit_winit::WindowEvent events to Servo where appropriate
+            match &event.window_event {
+                egui_winit::accesskit_winit::WindowEvent::InitialTreeRequested => {
+                    state.set_accessibility_active(true);
+                },
+                egui_winit::accesskit_winit::WindowEvent::ActionRequested(req) => {
+                    if req.target_tree != accesskit::TreeId::ROOT {
+                        // TODO(#4344): Forward action to Servo
+                    }
+                },
+                egui_winit::accesskit_winit::WindowEvent::AccessibilityDeactivated => {
+                    state.set_accessibility_active(false);
+                },
+            }
 
             if self
                 .gui

ports/servoshell/running_app_state.rs

@@ -216,6 +216,13 @@ pub(crate) struct RunningAppState {
 
     /// The currently focused [`ServoShellWindow`], if one is focused.
     focused_window: RefCell<Option<Rc<ServoShellWindow>>>,
+
+    /// Whether accessibility is active in servoshell.
+    ///
+    /// Set by the platform via AccessKit, and forwarded to existing and new WebViews via
+    /// [`WebView::set_accessibility_active()`], in [`Self::set_accessibility_active()`] and
+    /// and [`ServoShellWindow::create_toplevel_webview()`].
+    accessibility_active: Cell<bool>,
 }
 
 impl RunningAppState {
@@ -265,6 +272,7 @@ impl RunningAppState {
             exit_scheduled: Default::default(),
             user_content_manager,
             experimental_preferences_enabled,
+            accessibility_active: Cell::new(false),
         }
     }
 
@@ -274,10 +282,10 @@ impl RunningAppState {
         initial_url: Url,
     ) -> Rc<ServoShellWindow> {
         let window = Rc::new(ServoShellWindow::new(platform_window.clone()));
-        window.create_and_activate_toplevel_webview(self.clone(), initial_url);
         self.windows
             .borrow_mut()
             .insert(window.id(), window.clone());
+        window.create_and_activate_toplevel_webview(self.clone(), initial_url);
 
         // If the window already has platform focus, mark it as focused in our application state.
         if platform_window.has_platform_focus() {
@@ -661,6 +669,23 @@ impl RunningAppState {
             });
         }
     }
+
+    pub(crate) fn set_accessibility_active(&self, active: bool) {
+        let was_active = self.accessibility_active.replace(active);
+        if active == was_active {
+            return;
+        }
+
+        for window in self.windows().values() {
+            for (_, webview) in window.webviews() {
+                webview.set_accessibility_active(active);
+            }
+        }
+    }
+
+    pub(crate) fn accessibility_active(&self) -> bool {
+        self.accessibility_active.get()
+    }
 }
 
 impl WebViewDelegate for RunningAppState {

ports/servoshell/window.rs

@@ -74,6 +74,7 @@ impl ServoShellWindow {
         self.platform_window().id()
     }
 
+    /// Must be called *after* `self` is in `state.windows`, otherwise it will panic.
     pub(crate) fn create_and_activate_toplevel_webview(
         &self,
         state: Rc<RunningAppState>,
@@ -84,6 +85,7 @@ impl ServoShellWindow {
         webview
     }
 
+    /// Must be called *after* `self` is in `state.windows`, otherwise it will panic.
     pub(crate) fn create_toplevel_webview(&self, state: Rc<RunningAppState>, url: Url) -> WebView {
         let mut webview_builder =
             WebViewBuilder::new(state.servo(), self.platform_window.rendering_context())
@@ -103,6 +105,10 @@ impl ServoShellWindow {
         let webview = webview_builder.build();
         webview.notify_theme_change(self.platform_window.theme());
         self.add_webview(webview.clone());
+        // If `self` is not in `state.windows`, our notify_accessibility_tree_update() will panic.
+        if state.accessibility_active() {
+            webview.set_accessibility_active(true);
+        }
         webview
     }