webdriver: Focus browsing context when switch frame (#39086)

#38889 adds back the mechanism to focus the window when [switch to
window](https://w3c.github.io/webdriver/#switch-to-window). After that,
it causes many flaky TIMEOUT. Turns out the real reason is same as the
phenomenon which I thought was unrelated:
https://github.com/servo/servo/pull/38889#issuecomment-3217512339: we
need to focus the browsing context as well according to spec. This is
important for Servo because it relies on
f4dd2960b8/components/constellation/constellation_webview.rs (L64)
to determine which pipeline to send `InputEvent` to.

Testing: Before, there are 9 ~ 13 flaky results. [Before
1](https://github.com/yezhizhen/servo/actions/runs/17379170889), [Before
2](https://github.com/yezhizhen/servo/actions/runs/17392219417), [Before
3](https://github.com/yezhizhen/servo/actions/runs/17379172612).
Now there are 2 ~ 4 flaky results. [After
1](https://github.com/yezhizhen/servo/actions/runs/17394359570), [After
2](https://github.com/yezhizhen/servo/actions/runs/17394358218), [After
3](https://github.com/yezhizhen/servo/actions/runs/17394357400).
Fixes: https://github.com/servo/servo/pull/38889#issuecomment-3218600566
Fixes: #38906, which is last blocking point to enable WebDriver CI.

---------

Signed-off-by: Euclid Ye <euclid.ye@huawei.com>

components/constellation/constellation.rs

@@ -4524,6 +4524,9 @@ where
                 let is_open = self.browsing_contexts.contains_key(&browsing_context_id);
                 let _ = response_sender.send(is_open);
             },
+            WebDriverCommandMsg::FocusBrowsingContext(browsing_context_id) => {
+                self.handle_focus_remote_document_msg(browsing_context_id);
+            },
             // TODO: This should use the ScriptThreadMessage::EvaluateJavaScript command
             WebDriverCommandMsg::ScriptCommand(browsing_context_id, cmd) => {
                 let pipeline_id = if let Some(browsing_context) =

components/shared/embedder/webdriver.rs

@@ -173,6 +173,7 @@ pub enum WebDriverCommandMsg {
     ),
     GetAlertText(WebViewId, IpcSender<Result<String, ()>>),
     SendAlertText(WebViewId, String),
+    FocusBrowsingContext(BrowsingContextId),
 }
 
 #[derive(Debug, Deserialize, Serialize)]

components/webdriver_server/lib.rs

@@ -1214,8 +1214,14 @@ impl Handler {
                 self.handle_any_user_prompts(webview_id)?;
                 // Step 3. Set the current browsing context with session and
                 // session's current top-level browsing context.
+                let browsing_context_id = BrowsingContextId::from(webview_id);
                 self.session_mut()?
-                    .set_browsing_context_id(BrowsingContextId::from(webview_id));
+                    .set_browsing_context_id(browsing_context_id);
+
+                // Step 4. Update any implementation-specific state that would result from
+                // the user selecting session's current browsing context for interaction,
+                // without altering OS-level focus.
+                self.focus_browsing_context(browsing_context_id)?;
                 return Ok(WebDriverResponse::Void);
             },
             // id is a Number object
@@ -1260,6 +1266,10 @@ impl Handler {
             Ok(browsing_context_id) => {
                 self.session_mut()?
                     .set_browsing_context_id(browsing_context_id);
+                // Step 5. Update any implementation-specific state that would result from
+                // the user selecting session's current browsing context for interaction,
+                // without altering OS-level focus.
+                self.focus_browsing_context(browsing_context_id)?;
                 Ok(WebDriverResponse::Void)
             },
             Err(error) => Err(WebDriverError::new(error, "")),
@@ -1307,6 +1317,10 @@ impl Handler {
             Ok(browsing_context_id) => {
                 self.session_mut()?
                     .set_browsing_context_id(browsing_context_id);
+                // Step 4. Update any implementation-specific state that would result from
+                // the user selecting session's current browsing context for interaction,
+                // without altering OS-level focus.
+                self.focus_browsing_context(browsing_context_id)?;
                 Ok(WebDriverResponse::Void)
             },
             Err(error) => Err(WebDriverError::new(error, "")),
@@ -2506,6 +2520,12 @@ impl Handler {
         }
         Ok(())
     }
+
+    fn focus_browsing_context(&self, browsing_cotext_id: BrowsingContextId) -> WebDriverResult<()> {
+        self.send_message_to_embedder(WebDriverCommandMsg::FocusBrowsingContext(
+            browsing_cotext_id,
+        ))
+    }
 }
 
 impl WebDriverHandler<ServoExtensionRoute> for Handler {

ports/servoshell/desktop/app.rs

@@ -380,6 +380,9 @@ impl App {
                         running_state.set_pending_focus(focus_id, response_sender);
                     }
                 },
+                WebDriverCommandMsg::FocusBrowsingContext(..) => {
+                    running_state.servo().execute_webdriver_command(msg);
+                },
                 WebDriverCommandMsg::GetAllWebViews(response_sender) => {
                     let webviews = running_state.webviews().iter().map(|(id, _)| *id).collect();