Open a page in the container's browser, at a viewport you choose

The pane could only ever watch a browser something else had published.
This opens one: a URL and a viewport, launched inside the container and
bound so the pane picks it up. Two uses, one action — a sign-in page,
where the callback listener is *in* the container and the loop closes
with no host round trip and no auth bridge, and a dev server on container
loopback, which is how you watch a UI Claude is building.

Reachable from both places the question comes up: "Open a page…" in the
Browser tab, and an "In container" button on the terminal's URL prompt.

Verified first, because it decided the design: a second client cannot
join a bound browser. `chromium.connect()` against the published endpoint
times out in every URL form (`ws+unix://…`, with and without the trailing
path) — that socket speaks the dashboard's own transport, not the public
connect protocol. Whoever launches is therefore the only process that can
drive, so the helper is resident and holds the handle, and live resize
applies to pages we opened and never to `@playwright/mcp`'s. Those take
`--viewport-size` / `PLAYWRIGHT_MCP_VIEWPORT_SIZE` at launch, which the
docs now say.

The viewport is the interesting half. Resizing the *window* does nothing
to the page — the viewer is a CDP screencast, so a bigger window is the
same pixels drawn larger, which is why pages have been looking like they
were rendered small. `page.setViewportSize()` genuinely reflows: measured
against a `@media (max-width: 900px)` rule, it fires at 800×600 and
clears at 1440×900. Match-window mode pushes the pop-out's settled size
into it, debounced by generation counter because a drag emits `Resized`
continuously and each one costs a container exec.

Control is a polled JSON file in /tmp: no port, no second listener,
nothing added to the proxy's surface, and URLs travel as argv to `node`
so no shell ever parses one. A re-open with a helper already up
navigates instead of relaunching — otherwise the second page would throw
away the session the first one just signed into.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app/src-tauri/src/browser_view/popout.rs

@@ -27,6 +27,10 @@
 //! dead viewer is worse than no window. The reverse is not true; closing the
 //! window leaves the view running, and the pane takes it back into the tab.
 
+use std::collections::HashMap;
+use std::sync::{Mutex, OnceLock};
+use std::time::Duration;
+
 use serde::Serialize;
 use tauri::{AppHandle, Emitter, Manager, WebviewUrl, WebviewWindowBuilder, WindowEvent};
 
@@ -106,11 +110,17 @@ pub fn open(
         .map_err(|e| format!("Could not open the browser window: {}", e))?;
 
     // Closed from its own titlebar, this is the only thing that tells the pane
-    // to take the view back into the tab.
-    window.on_window_event(move |event| {
-        if matches!(event, WindowEvent::Destroyed) {
+    // to take the view back into the tab. `Resized` drives match-window mode —
+    // see `set_match_window`.
+    window.on_window_event(move |event| match event {
+        WindowEvent::Destroyed => {
+            set_match_window(&project_id_owned, false);
             emit(&app_for_event, &project_id_owned, PopoutState::CLOSED);
         }
+        WindowEvent::Resized(size) => {
+            on_resized(&app_for_event, &project_id_owned, size.width, size.height);
+        }
+        _ => {}
     });
 
     log::info!("Browser view: popped out for project {}", project_id);
@@ -170,6 +180,99 @@ pub fn set_always_on_top(app: &AppHandle, project_id: &str, on_top: bool) -> Res
     Ok(())
 }
 
+// ─────────────────────────────────────────────────────────────────────────────
+// Match-window mode
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// Projects whose pop-out is driving the page's viewport, and the generation of
+/// the latest resize for each — the debounce is "did anything else arrive while
+/// I slept?", which needs no timer to cancel.
+static MATCH_WINDOW: OnceLock<Mutex<HashMap<String, (bool, u64)>>> = OnceLock::new();
+
+/// How long the window has to stop moving before the page is resized.
+///
+/// A drag emits `Resized` continuously; each one costs a container exec, and
+/// Chromium relayouts the page. Settling first turns a drag into one resize.
+const RESIZE_SETTLE: Duration = Duration::from_millis(300);
+
+fn match_window_map() -> &'static Mutex<HashMap<String, (bool, u64)>> {
+    MATCH_WINDOW.get_or_init(|| Mutex::new(HashMap::new()))
+}
+
+/// Turn match-window mode on or off for a project.
+///
+/// Only ever affects a page **Triple-C opened** — a bound browser cannot be
+/// joined by a second client, so a page `@playwright/mcp` launched keeps
+/// whatever viewport it was given. See [`super::page`].
+pub fn set_match_window(project_id: &str, enabled: bool) {
+    let mut map = match_window_map().lock().unwrap_or_else(|e| e.into_inner());
+    let entry = map.entry(project_id.to_string()).or_insert((false, 0));
+    entry.0 = enabled;
+}
+
+pub fn match_window(project_id: &str) -> bool {
+    match_window_map()
+        .lock()
+        .unwrap_or_else(|e| e.into_inner())
+        .get(project_id)
+        .map(|(on, _)| *on)
+        .unwrap_or(false)
+}
+
+/// The pop-out's current inner size, for applying match-window immediately
+/// rather than only on the next drag.
+pub fn inner_size(app: &AppHandle, project_id: &str) -> Option<(u32, u32)> {
+    let window = app.get_webview_window(&window_label(project_id))?;
+    let size = window.inner_size().ok()?;
+    Some((size.width, size.height))
+}
+
+/// Debounce a resize, then push the settled size into the page's viewport.
+fn on_resized(app: &AppHandle, project_id: &str, width: u32, height: u32) {
+    let generation = {
+        let mut map = match_window_map().lock().unwrap_or_else(|e| e.into_inner());
+        let Some(entry) = map.get_mut(project_id) else {
+            return;
+        };
+        if !entry.0 {
+            return;
+        }
+        entry.1 += 1;
+        entry.1
+    };
+
+    let app = app.clone();
+    let project_id = project_id.to_string();
+    tauri::async_runtime::spawn(async move {
+        tokio::time::sleep(RESIZE_SETTLE).await;
+        // Superseded by a later resize: that one will do the work.
+        {
+            let map = match_window_map().lock().unwrap_or_else(|e| e.into_inner());
+            match map.get(&project_id) {
+                Some((true, latest)) if *latest == generation => {}
+                _ => return,
+            }
+        }
+
+        let state = app.state::<crate::AppState>();
+        let Some(container_id) = state
+            .projects_store
+            .get(&project_id)
+            .and_then(|p| p.container_id)
+        else {
+            return;
+        };
+        if let Err(e) = super::page::set_viewport(
+            &container_id,
+            super::page::Viewport::sane(width, height),
+        )
+        .await
+        {
+            log::debug!("Browser view: could not match the page to the window: {}", e);
+        }
+    });
+}
+
 fn emit(app: &AppHandle, project_id: &str, state: PopoutState) {
     let _ = app.emit(
         POPOUT_EVENT,
@@ -198,4 +301,38 @@ mod tests {
     fn distinct_projects_get_distinct_windows() {
         assert_ne!(window_label("alpha"), window_label("beta"));
     }
+
+    #[test]
+    fn match_window_is_off_until_asked_for_and_is_per_project() {
+        assert!(!match_window("mw-a"));
+        set_match_window("mw-a", true);
+        assert!(match_window("mw-a"));
+        // Another project's window must not start driving its page too.
+        assert!(!match_window("mw-b"));
+        set_match_window("mw-a", false);
+        assert!(!match_window("mw-a"));
+    }
+
+    #[test]
+    fn a_resize_supersedes_the_one_before_it() {
+        // The debounce is a generation counter, not a cancellable timer: only
+        // the newest resize of a drag survives to touch the container.
+        set_match_window("mw-gen", true);
+        let read = || {
+            match_window_map()
+                .lock()
+                .unwrap()
+                .get("mw-gen")
+                .map(|(_, g)| *g)
+                .unwrap()
+        };
+        let before = read();
+        {
+            let mut map = match_window_map().lock().unwrap();
+            let entry = map.get_mut("mw-gen").unwrap();
+            entry.1 += 1;
+        }
+        assert!(read() > before);
+        set_match_window("mw-gen", false);
+    }
 }