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>

CLAUDE.md

@@ -126,6 +126,23 @@ docker exec stdout → tokio task → emit("terminal-output-{sessionId}") → li
     viewer that no longer exists. It closes with `destroy()`, never `close()`, to stay clear of
     `CloseRequested`. The pane drops its iframe while popped out — two viewers can both *drive*
     the browser.
+  - **`page.rs` opens a page, which is the one thing the pane could not do.** A URL plus a
+    viewport: launch a browser in the container, `browser.bind()` it so the pane shows it, and
+    keep the handle. Serves auth (the OAuth callback listener is *in* the container, so a
+    container-side browser closes the loop with no host round trip and no auth bridge) and dev
+    servers on container loopback. **Verified: a second client cannot join a bound browser** —
+    `chromium.connect()` against the published endpoint times out in every URL form, because that
+    socket speaks the dashboard's transport, not the public connect protocol. So whoever launches
+    is the only process that can drive, which is why the helper is resident and why live resize
+    applies to pages *we* opened and never to `@playwright/mcp`'s (those take `--viewport-size` /
+    `PLAYWRIGHT_MCP_VIEWPORT_SIZE` at launch). Control is a polled JSON file in `/tmp` — no port,
+    no second listener — and a re-open with a helper already up *navigates* rather than
+    relaunching, so a session signed in on one page survives to the next.
+  - **Resizing the window does not resize the page.** The viewer is a CDP screencast: a bigger
+    window is the same pixels drawn larger. `page.setViewportSize()` is what reflows (measured
+    against a `@media (max-width: 900px)` rule), and match-window mode pushes the pop-out's
+    settled `Resized` size into it — debounced by generation counter, since a drag emits
+    continuously and each one costs a container exec.
   - **`lib.rs`'s `on_window_event` fires for every window and must stay guarded on
     `label() == "main"`.** Without that guard, closing a pop-out runs the app's shutdown: every
     container stopped, process exited.

HOW-TO-USE.md

@@ -280,11 +280,35 @@ Press **Start browser view** and the pane fills with Playwright's own dashboard,
 container and reached over a token-gated listener on your machine's loopback address. Nothing is
 exposed off the machine.
 
+#### Opening a page yourself
+
+**Open a page…** launches a browser inside the container at a URL and viewport you choose, and
+publishes it to this pane. Two uses:
+
+- **A sign-in page.** The callback the tool is waiting for is a listener *inside* the container, so
+  a container-side browser completes the login without anything crossing to your host browser.
+  When a long URL appears in a terminal, the prompt that offers to open it on your host now also
+  offers **In container**, which does the same thing in one click.
+- **A dev server.** `http://localhost:5173` inside the container is reachable with no port mapping
+  and nothing exposed to your network — which is how you watch a UI Claude is building, and click
+  around it yourself.
+
+The **viewport** is the page's own resolution, and it is not the same thing as the window size.
+The pane shows a video of the browser, so a bigger window draws the same pixels larger; changing
+the viewport is what makes the layout actually reflow. Pick a preset or type a size.
+
+Note the limit, because it is not obvious: a browser Claude opened through `@playwright/mcp` can
+be *watched* but not resized — a published browser admits only the client that launched it. Set
+its size with `PLAYWRIGHT_MCP_VIEWPORT_SIZE=1920x1080` in the project's environment variables
+instead.
+
 #### Watching it while you work
 
 Press **Open in own window** and the view moves out of the tab into a window of its own — put it on
 a second monitor, or turn on **Keep on top** and let it float above the app while you work in a
-terminal. This is a window change only: the browser and the view keep running throughout, so
+terminal. **Match window** goes further: the page's viewport follows the window as you drag it, so
+the pop-out becomes a responsive-design ruler. It applies to pages opened with **Open a page…**,
+for the reason above. This is a window change only: the browser and the view keep running throughout, so
 popping out and back costs nothing and interrupts nothing.
 
 While the view is in its own window the tab shows a placeholder rather than a second copy of it —

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

@@ -5,7 +5,7 @@
 use tauri::{AppHandle, State};
 
 use crate::browser_view::install::{self, BrowserSetupOutcome};
-use crate::browser_view::{manager, popout, BrowserViewState, BrowserViewStatus};
+use crate::browser_view::{manager, page, popout, BrowserViewState, BrowserViewStatus};
 use crate::AppState;
 
 /// Turn the pane on or off for a project.
@@ -164,6 +164,106 @@ pub async fn set_browser_view_popout_always_on_top(
     popout::set_always_on_top(&app_handle, &project_id, on_top)
 }
 
+/// Open a URL in a browser *inside* the container, published so the pane shows
+/// it.
+///
+/// Two uses, one action: an auth URL — where the OAuth callback listener is in
+/// the container too, so the loop closes without the host being involved at all
+/// — and a dev server on container loopback, which is how you watch a UI Claude
+/// is building.
+///
+/// The scheme allow-list mirrors the URL relay's: `http`/`https` only, so this
+/// can never be talked into opening `file:` on the container's filesystem.
+#[tauri::command]
+pub async fn open_page_in_container_browser(
+    project_id: String,
+    url: String,
+    width: u32,
+    height: u32,
+    state: State<'_, AppState>,
+) -> Result<page::PageState, String> {
+    let trimmed = url.trim();
+    if !(trimmed.starts_with("http://") || trimmed.starts_with("https://")) {
+        return Err("Only http:// and https:// URLs can be opened in the browser.".to_string());
+    }
+    let container_id = running_container(&state, &project_id, "opening a page").await?;
+    let detection = crate::browser_view::detect::detect(&container_id).await?;
+    page::open(
+        &container_id,
+        &detection,
+        trimmed,
+        page::Viewport::sane(width, height),
+    )
+    .await
+}
+
+/// Resize the page this opened. The pop-out's "match window" mode calls this on
+/// every settled resize, so it is deliberately cheap: one control-file write.
+#[tauri::command]
+pub async fn set_container_page_viewport(
+    project_id: String,
+    width: u32,
+    height: u32,
+    state: State<'_, AppState>,
+) -> Result<(), String> {
+    let container_id = running_container(&state, &project_id, "resizing the page").await?;
+    page::set_viewport(&container_id, page::Viewport::sane(width, height)).await
+}
+
+/// State of the page this opened, if any. Never fails: "no page" is an answer.
+#[tauri::command]
+pub async fn get_container_page_state(
+    project_id: String,
+    state: State<'_, AppState>,
+) -> Result<page::PageState, String> {
+    let Ok(container_id) = running_container(&state, &project_id, "reading the page").await else {
+        return Ok(page::PageState::default());
+    };
+    Ok(page::state(&container_id).await)
+}
+
+/// Close the page this opened, leaving the view itself running.
+#[tauri::command]
+pub async fn close_container_page(
+    project_id: String,
+    state: State<'_, AppState>,
+) -> Result<(), String> {
+    let container_id = running_container(&state, &project_id, "closing the page").await?;
+    page::close(&container_id).await;
+    Ok(())
+}
+
+/// Make the page track the pop-out window's size as it is dragged.
+///
+/// Only affects a page **this app opened**: a bound browser admits no second
+/// client, so one `@playwright/mcp` launched keeps the viewport it was given.
+/// Turning it on applies the window's current size immediately, so the toggle
+/// has a visible effect without waiting for a drag.
+#[tauri::command]
+pub async fn set_browser_view_match_window(
+    project_id: String,
+    enabled: bool,
+    app_handle: AppHandle,
+    state: State<'_, AppState>,
+) -> Result<(), String> {
+    popout::set_match_window(&project_id, enabled);
+    if !enabled {
+        return Ok(());
+    }
+    let Some((width, height)) = popout::inner_size(&app_handle, &project_id) else {
+        return Ok(());
+    };
+    let container_id = running_container(&state, &project_id, "matching the window").await?;
+    page::set_viewport(&container_id, page::Viewport::sane(width, height)).await
+}
+
+/// Whether match-window mode is on. Read on mount, like the rest of the
+/// pop-out's state — the pane is unmounted whenever another sub-tab is shown.
+#[tauri::command]
+pub async fn get_browser_view_match_window(project_id: String) -> Result<bool, String> {
+    Ok(popout::match_window(&project_id))
+}
+
 /// The project's container, or a sentence saying why there isn't one.
 ///
 /// Every command here needs a *running* container, and every one of them used

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

@@ -64,6 +64,7 @@
 pub mod commands;
 pub mod detect;
 pub mod install;
+pub mod page;
 pub mod popout;
 pub mod proxy;
 

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

@@ -0,0 +1,365 @@
+//! Open a page in the container's browser, and resize it while it runs.
+//!
+//! The pane [watches](super) browsers something else published. This opens one:
+//! the user hands it a URL, it launches a browser inside the container,
+//! publishes it with `browser.bind()` so the pane picks it up, and holds the
+//! handle so the page can be navigated and **resized** afterwards.
+//!
+//! ## Why the handle has to be held
+//!
+//! Verified against a real bound browser: a second client cannot join one.
+//! `chromium.connect()` against the published endpoint times out in every URL
+//! form — the descriptor's socket speaks the dashboard's own transport, not the
+//! public connect protocol. So whoever launches the browser is the only process
+//! that can ever drive it. That is the whole reason this helper is a resident
+//! process rather than a one-shot `node -e` that exits.
+//!
+//! It also draws the line for the feature: pages *this* opens can be resized
+//! live; a browser `@playwright/mcp` launched can only be watched, and its size
+//! is whatever `--viewport-size` it was given.
+//!
+//! ## Control channel
+//!
+//! A JSON file in `/tmp`, polled by the helper. No port, no second listener, no
+//! addition to the proxy's attack surface — and it composes with the one exec
+//! path this codebase already has. Writes go through `node -e` rather than
+//! shell redirection so a URL never touches a shell.
+//!
+//! ## Viewport, and why it is the interesting part
+//!
+//! `page.setViewportSize()` genuinely reflows: measured on a page carrying a
+//! `@media (max-width: 900px)` rule, the rule fires at 800×600 and clears at
+//! 1440×900. Resizing the *window* the pane lives in does nothing of the sort —
+//! the viewer is a CDP screencast, so a bigger window is the same pixels drawn
+//! larger. This is what makes the pop-out usable as a responsive-design ruler.
+
+use serde::{Deserialize, Serialize};
+
+use crate::docker::exec::exec_oneshot_as;
+
+use super::detect::PlaywrightDetection;
+
+/// Control file the helper polls, and the state file it writes back.
+const CONTROL_PATH: &str = "/tmp/triple-c-page-control.json";
+const STATE_PATH: &str = "/tmp/triple-c-page-state.json";
+/// Where the detached helper's own output goes, so a failed start has a trail.
+const HELPER_LOG: &str = "/tmp/triple-c-page.log";
+
+/// How long to wait for the helper to report that the page is up.
+const READY_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(45);
+/// Navigating a browser that is already up. One page load, not a cold start.
+const REUSE_TIMEOUT: std::time::Duration = std::time::Duration::from_secs(35);
+const READY_POLL: std::time::Duration = std::time::Duration::from_millis(400);
+
+/// A viewport, in CSS pixels.
+#[derive(Debug, Clone, Copy, Serialize, Deserialize, PartialEq, Eq)]
+pub struct Viewport {
+    pub width: u32,
+    pub height: u32,
+}
+
+impl Viewport {
+    /// Clamped to something a browser will accept. A window dragged to nothing
+    /// must not ask Chromium for a zero-width page.
+    pub fn sane(width: u32, height: u32) -> Self {
+        Self {
+            width: width.clamp(200, 7680),
+            height: height.clamp(200, 4320),
+        }
+    }
+}
+
+/// What the helper reports about itself.
+#[derive(Debug, Clone, Deserialize, Serialize, Default)]
+pub struct PageState {
+    #[serde(default)]
+    pub ready: bool,
+    #[serde(default)]
+    pub url: Option<String>,
+    #[serde(default)]
+    pub viewport: Option<Viewport>,
+    #[serde(default)]
+    pub error: Option<String>,
+}
+
+/// Open `url` in a freshly launched, bound browser.
+///
+/// Replaces any page this opened before: one helper per container, because the
+/// pane shows one browser and a second would just compete for the pane.
+pub async fn open(
+    container_id: &str,
+    detection: &PlaywrightDetection,
+    url: &str,
+    viewport: Viewport,
+) -> Result<PageState, String> {
+    let core = detection.playwright_path.as_deref().ok_or_else(|| {
+        "Playwright isn't installed in this container — set it up from the Browser tab first."
+            .to_string()
+    })?;
+    // The directory of the resolved manifest is what `require()` wants.
+    let core_dir = core.trim_end_matches("/package.json");
+
+    // The executable is passed explicitly rather than left to Playwright's
+    // revision lookup: a container can hold browsers a given copy will not
+    // launch (see `detect::revision_skew`), and this is the one place we know
+    // which binary is actually on disk.
+    let executable = detection
+        .chromium_executable
+        .as_deref()
+        .filter(|_| detection.chromium_executable_exists);
+
+    // Reuse a helper that is already up. Relaunching would throw away the
+    // browser's cookies and storage — which for the auth case means signing in
+    // again to reach the second page, having just signed in on the first.
+    if state(container_id).await.ready {
+        set_viewport(container_id, viewport).await?;
+        navigate(container_id, url).await?;
+        if let Some(state) = wait_for_url(container_id, url).await {
+            return Ok(state);
+        }
+        // It stopped answering; fall through and start a fresh one.
+    }
+
+    close(container_id).await;
+
+    let config = serde_json::json!({
+        "core": core_dir,
+        "executable": executable,
+        "url": url,
+        "viewport": viewport,
+        "control": CONTROL_PATH,
+        "state": STATE_PATH,
+    });
+    let script = format!("const CFG={};{}", config, HELPER);
+
+    // Detached, for the same reason the viewer is: the process has to outlive
+    // the exec that started it, or the page closes the moment we return.
+    let launcher = format!(
+        "cd /workspace 2>/dev/null || true; rm -f {} {}; nohup node -e {} >{} 2>&1 &",
+        STATE_PATH,
+        CONTROL_PATH,
+        shell_quote(&script),
+        HELPER_LOG
+    );
+    exec_oneshot_as(
+        container_id,
+        "claude",
+        vec!["sh".to_string(), "-c".to_string(), launcher],
+        Vec::new(),
+    )
+    .await
+    .map_err(|e| format!("Could not start the browser helper: {}", e))?;
+
+    wait_until_ready(container_id).await
+}
+
+/// Resize the open page. Cheap enough to call from a window-resize handler.
+pub async fn set_viewport(container_id: &str, viewport: Viewport) -> Result<(), String> {
+    write_control(
+        container_id,
+        serde_json::json!({ "viewport": viewport }).to_string(),
+    )
+    .await
+}
+
+/// Navigate the open page without relaunching the browser.
+pub async fn navigate(container_id: &str, url: &str) -> Result<(), String> {
+    write_control(container_id, serde_json::json!({ "url": url }).to_string()).await
+}
+
+/// Ask the helper to shut down. Best effort: a container that has none is the
+/// normal case, and the caller is usually about to start one anyway.
+pub async fn close(container_id: &str) {
+    let _ = write_control(container_id, serde_json::json!({ "close": true }).to_string()).await;
+}
+
+/// Current state, or a default when no helper has ever run here.
+pub async fn state(container_id: &str) -> PageState {
+    let script = format!(
+        "try{{process.stdout.write(require('fs').readFileSync('{}','utf8'));}}catch(e){{}}",
+        STATE_PATH
+    );
+    let Ok((out, _)) = exec_oneshot_as(
+        container_id,
+        "claude",
+        vec!["node".to_string(), "-e".to_string(), script],
+        Vec::new(),
+    )
+    .await
+    else {
+        return PageState::default();
+    };
+    serde_json::from_str(out.trim()).unwrap_or_default()
+}
+
+/// Write the control file through Node rather than a shell redirect, so a URL
+/// is never interpreted by `sh`.
+async fn write_control(container_id: &str, json: String) -> Result<(), String> {
+    let script = format!(
+        "require('fs').writeFileSync('{}',process.argv[1]);",
+        CONTROL_PATH
+    );
+    exec_oneshot_as(
+        container_id,
+        "claude",
+        vec!["node".to_string(), "-e".to_string(), script, json],
+        Vec::new(),
+    )
+    .await
+    .map(|_| ())
+    .map_err(|e| format!("Could not reach the browser helper: {}", e))
+}
+
+/// Wait for a *running* helper to report the URL we just asked it for.
+///
+/// Bounded much tighter than a cold start: the browser is already up, so this
+/// is one navigation. `None` means it stopped answering, and the caller starts
+/// a fresh helper rather than reporting a page that isn't there.
+async fn wait_for_url(container_id: &str, url: &str) -> Option<PageState> {
+    let deadline = std::time::Instant::now() + REUSE_TIMEOUT;
+    loop {
+        let state = state(container_id).await;
+        if state.ready && state.url.as_deref() == Some(url) {
+            return Some(state);
+        }
+        if std::time::Instant::now() >= deadline {
+            return None;
+        }
+        tokio::time::sleep(READY_POLL).await;
+    }
+}
+
+/// Poll the state file until the helper says the page is up, or says why not.
+async fn wait_until_ready(container_id: &str) -> Result<PageState, String> {
+    let deadline = std::time::Instant::now() + READY_TIMEOUT;
+    loop {
+        let state = state(container_id).await;
+        if let Some(error) = state.error.clone() {
+            return Err(error);
+        }
+        if state.ready {
+            return Ok(state);
+        }
+        if std::time::Instant::now() >= deadline {
+            return Err(format!(
+                "The browser didn't come up within {}s. Its log is at {} inside the container.",
+                READY_TIMEOUT.as_secs(),
+                HELPER_LOG
+            ));
+        }
+        tokio::time::sleep(READY_POLL).await;
+    }
+}
+
+/// Single-quote for `sh`, the same way [`super`] does for the viewer's paths.
+fn shell_quote(s: &str) -> String {
+    format!("'{}'", s.replace('\'', r"'\''"))
+}
+
+/// The resident helper, appended to a `const CFG={…};` prelude.
+///
+/// Deliberately one string passed as a single `argv` element — no shell parsing
+/// of any part of it, exactly like `detect`'s probe. It launches, binds, and
+/// then polls the control file; every failure path writes the state file, so a
+/// helper that dies during startup is reported rather than waited out.
+const HELPER: &str = concat!(
+    r#"const fs=require('fs');"#,
+    r#"const {chromium}=require(CFG.core);"#,
+    r#"const write=(o)=>{try{fs.writeFileSync(CFG.state,JSON.stringify(o));}catch(e){}};"#,
+    r#"const fail=(e)=>{write({ready:false,error:String(e&&e.message||e)});process.exit(1);};"#,
+    r#"process.on('unhandledRejection',fail);"#,
+    r#"(async()=>{"#,
+    // `chromiumSandbox:false` because the container has no user namespaces to
+    // give Chromium; headless because there is no display, which is also the
+    // only mode the dashboard can screencast anyway.
+    r#"const opts={headless:true,chromiumSandbox:false};"#,
+    r#"if(CFG.executable)opts.executablePath=CFG.executable;"#,
+    r#"const browser=await chromium.launch(opts);"#,
+    r#"const ctx=await browser.newContext({viewport:CFG.viewport});"#,
+    r#"const page=await ctx.newPage();"#,
+    // Bind before navigating: the pane should show the page loading rather than
+    // appearing once it is done.
+    r#"await browser.bind('claude',{metadata:{source:'triple-c'}});"#,
+    r#"let current=CFG.url,viewport=CFG.viewport;"#,
+    r#"const report=()=>write({ready:true,url:current,viewport});"#,
+    r#"try{await page.goto(CFG.url,{waitUntil:'domcontentloaded',timeout:30000});}catch(e){}"#,
+    r#"report();"#,
+    // The control loop. A poll, not a watcher: `fs.watch` misses writes on some
+    // filesystems and this costs nothing at 4 Hz.
+    r#"setInterval(async()=>{let c;try{c=JSON.parse(fs.readFileSync(CFG.control,'utf8'));}catch(e){return;}"#,
+    r#"try{fs.unlinkSync(CFG.control);}catch(e){}"#,
+    r#"if(c.close){await browser.close().catch(()=>{});write({ready:false});process.exit(0);}"#,
+    r#"if(c.viewport){viewport=c.viewport;await page.setViewportSize(c.viewport).catch(()=>{});}"#,
+    r#"if(c.url&&c.url!==current){current=c.url;await page.goto(c.url,{waitUntil:'domcontentloaded',timeout:30000}).catch(()=>{});}"#,
+    r#"report();},250);"#,
+    // A browser that dies (crash, or the user closing the last page) must not
+    // leave a helper claiming a live page.
+    r#"browser.on('disconnected',()=>{write({ready:false});process.exit(0);});"#,
+    r#"})().catch(fail);"#,
+);
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn the_helper_is_one_argv_element_with_no_shell_hazards() {
+        // Same rule as the detect probe: it is passed as a single argument, so
+        // it must contain neither a newline nor a single quote that would end
+        // the quoting `open` wraps it in.
+        assert!(!HELPER.contains('\n'), "{}", HELPER);
+        assert!(HELPER.contains("chromium.launch"), "{}", HELPER);
+    }
+
+    #[test]
+    fn the_helper_binds_so_the_pane_can_see_the_page() {
+        // Without this the page opens and the pane shows nothing — the whole
+        // feature hinges on the browser being published.
+        assert!(HELPER.contains("browser.bind('claude'"), "{}", HELPER);
+    }
+
+    #[test]
+    fn the_helper_reports_startup_failures_instead_of_hanging() {
+        // `wait_until_ready` polls the state file; a helper that dies silently
+        // would turn every failure into a 45-second timeout.
+        assert!(HELPER.contains("unhandledRejection"), "{}", HELPER);
+        assert!(HELPER.contains("error:String"), "{}", HELPER);
+    }
+
+    #[test]
+    fn a_viewport_is_clamped_to_something_a_browser_accepts() {
+        assert_eq!(Viewport::sane(0, 0), Viewport { width: 200, height: 200 });
+        assert_eq!(
+            Viewport::sane(99_999, 99_999),
+            Viewport { width: 7680, height: 4320 }
+        );
+        assert_eq!(
+            Viewport::sane(1440, 900),
+            Viewport { width: 1440, height: 900 }
+        );
+    }
+
+    #[test]
+    fn a_url_is_never_parsed_by_a_shell() {
+        // The launcher runs through `sh -c`, so the script is quoted with the
+        // POSIX close-escape-reopen form: the embedded quote becomes `'\''`,
+        // which leaves the `;rm` inside the string rather than starting a new
+        // command. (A naive "the output must not contain ';rm'" check fails
+        // here and would be wrong — that substring is *inside* the quoting.)
+        assert_eq!(
+            shell_quote("http://x/?a=1&b=2';rm -rf /"),
+            r"'http://x/?a=1&b=2'\'';rm -rf /'"
+        );
+        // The control channel doesn't go near a shell at all: the JSON travels
+        // as an argv element to `node`.
+        assert!(!HELPER.contains("exec("), "{}", HELPER);
+    }
+
+    #[test]
+    fn state_defaults_to_not_ready_rather_than_failing() {
+        // An empty/absent state file is the normal case before anything runs.
+        let s: PageState = serde_json::from_str("{}").unwrap();
+        assert!(!s.ready);
+        assert!(s.error.is_none());
+    }
+}

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.
+    // to take the view back into the tab. `Resized` drives match-window mode —
-    window.on_window_event(move |event| {
+    // see `set_match_window`.
-        if matches!(event, WindowEvent::Destroyed) {
+    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);
+    }
 }

app/src-tauri/src/lib.rs

@@ -440,6 +440,12 @@ pub fn run() {
             browser_view::commands::close_browser_view_popout,
             browser_view::commands::get_browser_view_popout_state,
             browser_view::commands::set_browser_view_popout_always_on_top,
+            browser_view::commands::open_page_in_container_browser,
+            browser_view::commands::set_container_page_viewport,
+            browser_view::commands::get_container_page_state,
+            browser_view::commands::close_container_page,
+            browser_view::commands::set_browser_view_match_window,
+            browser_view::commands::get_browser_view_match_window,
             // Shared Claude Code auth token
             commands::auth_token_commands::acquire_claude_token,
             commands::auth_token_commands::submit_claude_token_code,

app/src/components/projects/home/BrowserTab.test.tsx

@@ -18,6 +18,10 @@ const closeBrowserViewPopout = vi.fn<(id: string) => Promise<void>>();
 const getBrowserViewPopoutState =
   vi.fn<() => Promise<{ open: boolean; always_on_top: boolean }>>();
 const setBrowserViewPopoutAlwaysOnTop = vi.fn<(id: string, onTop: boolean) => Promise<void>>();
+const openPageInContainerBrowser =
+  vi.fn<(id: string, url: string, w: number, h: number) => Promise<{ error: string | null }>>();
+const setBrowserViewMatchWindow = vi.fn<(id: string, on: boolean) => Promise<void>>();
+const getBrowserViewMatchWindow = vi.fn<() => Promise<boolean>>();
 const pushToast = vi.fn();
 const setContainerProgress = vi.fn();
 
@@ -32,6 +36,10 @@ vi.mock("../../../lib/tauri-commands", () => ({
   getBrowserViewPopoutState: () => getBrowserViewPopoutState(),
   setBrowserViewPopoutAlwaysOnTop: (id: string, onTop: boolean) =>
     setBrowserViewPopoutAlwaysOnTop(id, onTop),
+  openPageInContainerBrowser: (id: string, url: string, w: number, h: number) =>
+    openPageInContainerBrowser(id, url, w, h),
+  setBrowserViewMatchWindow: (id: string, on: boolean) => setBrowserViewMatchWindow(id, on),
+  getBrowserViewMatchWindow: () => getBrowserViewMatchWindow(),
 }));
 
 vi.mock("@tauri-apps/api/event", () => ({
@@ -130,6 +138,9 @@ beforeEach(() => {
   openBrowserViewPopout.mockResolvedValue(undefined);
   closeBrowserViewPopout.mockResolvedValue(undefined);
   setBrowserViewPopoutAlwaysOnTop.mockResolvedValue(undefined);
+  setBrowserViewMatchWindow.mockResolvedValue(undefined);
+  getBrowserViewMatchWindow.mockResolvedValue(false);
+  openPageInContainerBrowser.mockResolvedValue({ error: null });
 });
 
 const LIVE: BrowserViewStatus = {
@@ -494,6 +505,58 @@ describe("BrowserTab", () => {
     expect(await screen.findByTitle("Playwright browser view for api-server")).toBeInTheDocument();
   });
 
+  it("opens a page in the container’s browser at the chosen viewport", async () => {
+    await renderLive();
+
+    await act(async () => {
+      fireEvent.click(screen.getByRole("button", { name: /open a page/i }));
+    });
+    fireEvent.change(screen.getByLabelText(/^URL$/i), {
+      target: { value: "http://localhost:5173" },
+    });
+    await act(async () => {
+      fireEvent.click(screen.getByRole("button", { name: "1920 × 1080" }));
+    });
+    await act(async () => {
+      fireEvent.click(screen.getByRole("button", { name: /open page/i }));
+    });
+
+    expect(openPageInContainerBrowser).toHaveBeenCalledWith(
+      "p1",
+      "http://localhost:5173",
+      1920,
+      1080,
+    );
+  });
+
+  it("refuses a URL scheme the backend would reject, before the round trip", async () => {
+    await renderLive();
+    await act(async () => {
+      fireEvent.click(screen.getByRole("button", { name: /open a page/i }));
+    });
+    fireEvent.change(screen.getByLabelText(/^URL$/i), {
+      target: { value: "file:///etc/passwd" },
+    });
+
+    expect(screen.getByRole("button", { name: /open page/i })).toBeDisabled();
+    expect(screen.getByText(/Only http:\/\/ and https:\/\//)).toBeInTheDocument();
+    expect(openPageInContainerBrowser).not.toHaveBeenCalled();
+  });
+
+  it("offers match-window only once the view is in its own window", async () => {
+    await renderLive();
+    expect(screen.queryByRole("switch", { name: "Match window" })).toBeNull();
+
+    await act(async () => {
+      fireEvent.click(screen.getByRole("button", { name: /own window/i }));
+    });
+    await act(async () => {
+      fireEvent.click(screen.getByRole("switch", { name: "Match window" }));
+    });
+
+    expect(setBrowserViewMatchWindow).toHaveBeenCalledWith("p1", true);
+  });
+
   it("says why the window wouldn’t open instead of pretending it did", async () => {
     await renderLive();
     openBrowserViewPopout.mockRejectedValue("no display");

app/src/components/projects/home/BrowserTab.tsx

@@ -15,12 +15,16 @@ import {
   getBrowserViewStatus,
   installBrowserViewBrowser,
   installBrowserViewSupport,
+  getBrowserViewMatchWindow,
   getBrowserViewPopoutState,
   openBrowserViewPopout,
+  openPageInContainerBrowser,
   setBrowserViewEnabled,
+  setBrowserViewMatchWindow,
   setBrowserViewPopoutAlwaysOnTop,
 } from "../../../lib/tauri-commands";
 import { useAppState } from "../../../store/appState";
+import OpenPageDialog from "./OpenPageDialog";
 import AccordionSection from "../../ui/AccordionSection";
 import Button from "../../ui/Button";
 import StatusIndicator from "../../ui/StatusIndicator";
@@ -80,6 +84,10 @@ export default function BrowserTab({ project, active }: Props) {
    */
   const [poppedOut, setPoppedOut] = useState<boolean | null>(null);
   const [onTop, setOnTop] = useState(false);
+  /** The "open a page" dialog, and the request it is running. */
+  const [matchWindow, setMatchWindow] = useState(false);
+  const [askPage, setAskPage] = useState(false);
+  const [openingPage, setOpeningPage] = useState(false);
   const pushToast = useAppState((s) => s.pushToast);
   const setContainerProgress = useAppState((s) => s.setContainerProgress);
   const progress = useAppState((s) => s.containerProgress[project.id]);
@@ -137,6 +145,9 @@ export default function BrowserTab({ project, active }: Props) {
       // Unreachable in practice, but a pane stuck at "not asked yet" would
       // never show the view at all — so fail towards the tab.
       .catch(() => mounted.current && setPoppedOut(false));
+    getBrowserViewMatchWindow(projectId)
+      .then((on) => mounted.current && setMatchWindow(on))
+      .catch(() => {});
     getBrowserViewStatus(projectId)
       .then((s) => mounted.current && setStatus(s))
       .catch(() => {});
@@ -219,6 +230,55 @@ export default function BrowserTab({ project, active }: Props) {
     [projectId, pushToast],
   );
 
+  /**
+   * Open a URL in a browser inside the container.
+   *
+   * The pane only ever *watched* browsers something else published; this is the
+   * one action that opens one. It also means the page can be resized later —
+   * whoever launches a bound browser is the only process that can drive it.
+   */
+  const openPage = useCallback(
+    async (url: string, width: number, height: number) => {
+      setOpeningPage(true);
+      try {
+        const result = await openPageInContainerBrowser(projectId, url, width, height);
+        if (!mounted.current) return;
+        setAskPage(false);
+        if (result.error) {
+          pushToast({ kind: "error", message: "The page didn’t open", detail: result.error });
+        } else {
+          pushToast({ kind: "success", message: `Opened ${url} at ${width}×${height}` });
+        }
+      } catch (e) {
+        pushToast({
+          kind: "error",
+          message: "Could not open the page in the container’s browser",
+          detail: String(e),
+        });
+      } finally {
+        if (mounted.current) setOpeningPage(false);
+      }
+    },
+    [projectId, pushToast],
+  );
+
+  const toggleMatchWindow = useCallback(
+    async (next: boolean) => {
+      setMatchWindow(next);
+      try {
+        await setBrowserViewMatchWindow(projectId, next);
+      } catch (e) {
+        if (mounted.current) setMatchWindow(!next);
+        pushToast({
+          kind: "error",
+          message: "Could not match the page to the window",
+          detail: String(e),
+        });
+      }
+    },
+    [projectId, pushToast],
+  );
+
   /** Run one install. Every path clears the progress line it started. */
   const install = useCallback(
     async (which: Exclude<SetupJob, null>) => {
@@ -326,11 +386,25 @@ export default function BrowserTab({ project, active }: Props) {
             <Toggle checked={onTop} onChange={toggleOnTop} label="Keep on top" />
           </span>
         )}
+        {live && poppedOut === true && (
+          <span
+            className="flex items-center gap-1.5 text-xs text-[var(--text-secondary)]"
+            title="Resize the page itself as the window is dragged, so the layout actually reflows. Applies to pages opened from here."
+          >
+            Match window
+            <Toggle checked={matchWindow} onChange={toggleMatchWindow} label="Match window" />
+          </span>
+        )}
         {live && poppedOut === false && (
           <Button size="md" onClick={() => setReloadKey((k) => k + 1)}>
             Reload
           </Button>
         )}
+        {live && (
+          <Button size="md" onClick={() => setAskPage(true)}>
+            Open a page…
+          </Button>
+        )}
         {live && poppedOut !== null && (
           <Button size="md" onClick={poppedOut ? popIn : popOut}>
             {poppedOut ? "Put back in tab" : "Open in own window"}
@@ -415,6 +489,14 @@ export default function BrowserTab({ project, active }: Props) {
           )}
         </div>
       )}
+
+      {askPage && (
+        <OpenPageDialog
+          busy={openingPage}
+          onOpen={openPage}
+          onClose={() => setAskPage(false)}
+        />
+      )}
     </div>
   );
 }

app/src/components/projects/home/OpenPageDialog.tsx

@@ -0,0 +1,148 @@
+import { useState } from "react";
+import Modal from "../../ui/Modal";
+import Button from "../../ui/Button";
+
+/**
+ * Viewport presets. These are the *page's* resolution, not the window's — the
+ * pane is a screencast, so a bigger window shows the same pixels drawn larger
+ * while this is what actually reflows the layout.
+ */
+const PRESETS: { label: string; width: number; height: number }[] = [
+  { label: "1280 × 720", width: 1280, height: 720 },
+  { label: "1920 × 1080", width: 1920, height: 1080 },
+  { label: "1440 × 900", width: 1440, height: 900 },
+  { label: "390 × 844 (phone)", width: 390, height: 844 },
+];
+
+interface Props {
+  /** Prefilled URL — an auth URL from the terminal, or the last one used. */
+  initialUrl?: string;
+  initialWidth?: number;
+  initialHeight?: number;
+  busy?: boolean;
+  onOpen: (url: string, width: number, height: number) => void;
+  onClose: () => void;
+}
+
+/**
+ * Ask for a URL and a viewport, then open it in the container's browser.
+ *
+ * Deliberately modal and short-lived — the convention for a task with one
+ * question and one button. The URL is not opened here; the caller runs the
+ * command so failures land in its toast.
+ */
+export default function OpenPageDialog({
+  initialUrl = "",
+  initialWidth = 1280,
+  initialHeight = 720,
+  busy = false,
+  onOpen,
+  onClose,
+}: Props) {
+  const [url, setUrl] = useState(initialUrl);
+  const [width, setWidth] = useState(initialWidth);
+  const [height, setHeight] = useState(initialHeight);
+
+  const trimmed = url.trim();
+  // Mirrors the backend's allow-list, so the error arrives before the click
+  // rather than after a round trip.
+  const valid = /^https?:\/\/\S+$/i.test(trimmed);
+
+  return (
+    <Modal
+      title="Open a page in the container's browser"
+      onClose={onClose}
+      footer={
+        <>
+          <Button size="md" onClick={onClose} disabled={busy}>
+            Cancel
+          </Button>
+          <Button
+            size="md"
+            variant="primary"
+            disabled={!valid || busy}
+            onClick={() => onOpen(trimmed, width, height)}
+          >
+            {busy ? "Opening…" : "Open page"}
+          </Button>
+        </>
+      }
+    >
+      <div className="space-y-4">
+        <p className="text-[13px] text-[var(--text-secondary)] leading-relaxed">
+          Launches a browser <em>inside</em> this container and publishes it to the
+          Browser tab. Use it for a sign-in page — the callback listener is in the
+          container too, so the login completes without involving your host browser —
+          or for a dev server on container loopback.
+        </p>
+
+        <label className="block">
+          <span className="text-xs text-[var(--text-secondary)]">URL</span>
+          <input
+            autoFocus
+            value={url}
+            onChange={(e) => setUrl(e.target.value)}
+            onKeyDown={(e) => {
+              if (e.key === "Enter" && valid && !busy) onOpen(trimmed, width, height);
+            }}
+            placeholder="http://localhost:5173"
+            spellCheck={false}
+            className="mt-1 w-full px-2 py-1.5 bg-[var(--bg-primary)] border border-[var(--border-color)] rounded-[var(--radius-control)] text-[13px] font-mono text-[var(--text-primary)]"
+          />
+          {trimmed !== "" && !valid && (
+            <span className="mt-1 block text-xs text-[var(--error)]">
+              Only http:// and https:// URLs can be opened.
+            </span>
+          )}
+        </label>
+
+        <div>
+          <span className="text-xs text-[var(--text-secondary)]">Viewport</span>
+          <div className="mt-1 flex flex-wrap gap-1.5">
+            {PRESETS.map((p) => {
+              const active = p.width === width && p.height === height;
+              return (
+                <button
+                  key={p.label}
+                  type="button"
+                  aria-pressed={active}
+                  onClick={() => {
+                    setWidth(p.width);
+                    setHeight(p.height);
+                  }}
+                  className={`px-2 py-1 text-xs rounded-[var(--radius-control)] border transition-colors ${
+                    active
+                      ? "border-[var(--accent)] bg-[var(--accent-muted)] text-[var(--accent)]"
+                      : "border-[var(--border-color)] text-[var(--text-secondary)] hover:text-[var(--text-primary)]"
+                  }`}
+                >
+                  {p.label}
+                </button>
+              );
+            })}
+          </div>
+          <div className="mt-2 flex items-center gap-2">
+            <input
+              type="number"
+              aria-label="Viewport width"
+              value={width}
+              min={200}
+              onChange={(e) => setWidth(Number(e.target.value))}
+              className="w-24 px-2 py-1 bg-[var(--bg-primary)] border border-[var(--border-color)] rounded-[var(--radius-control)] text-xs text-[var(--text-primary)]"
+            />
+            <span aria-hidden="true" className="text-xs text-[var(--text-secondary)]">×</span>
+            <input
+              type="number"
+              aria-label="Viewport height"
+              value={height}
+              min={200}
+              onChange={(e) => setHeight(Number(e.target.value))}
+              className="w-24 px-2 py-1 bg-[var(--bg-primary)] border border-[var(--border-color)] rounded-[var(--radius-control)] text-xs text-[var(--text-primary)]"
+            />
+            <span className="text-xs text-[var(--text-secondary)]">CSS pixels</span>
+          </div>
+        </div>
+      </div>
+    </Modal>
+  );
+}

app/src/components/terminal/TerminalView.tsx

@@ -7,7 +7,11 @@ import { openUrl } from "@tauri-apps/plugin-opener";
 import "@xterm/xterm/css/xterm.css";
 import { useTerminal } from "../../hooks/useTerminal";
 import { useAppState } from "../../store/appState";
-import { awsSsoRefresh, uploadHostFileToTerminal } from "../../lib/tauri-commands";
+import {
+  awsSsoRefresh,
+  openPageInContainerBrowser,
+  uploadHostFileToTerminal,
+} from "../../lib/tauri-commands";
 import { getCurrentWebview } from "@tauri-apps/api/webview";
 import { UrlDetector } from "../../lib/urlDetector";
 import {
@@ -529,6 +533,46 @@ export default function TerminalView({ sessionId, active }: Props) {
     openUrl(safe).catch((e) => console.error("Failed to open URL:", e));
   }, [urlPrompt]);
 
+  /**
+   * Open the prompted URL in the container's own browser instead of the host's.
+   *
+   * For a sign-in this is the shorter path: the callback listener the tool is
+   * waiting on is inside the container, so a container-side browser closes the
+   * loop with nothing crossing to the host. The page is published to the
+   * project's Browser tab, which is where the user completes it by hand.
+   */
+  const handleOpenUrlInContainer = useCallback(() => {
+    if (!urlPrompt) return;
+    const safe = sanitizeRelayUrl(urlPrompt.url);
+    setUrlPrompt(null);
+    if (!safe) {
+      console.warn("Refusing to open a URL that failed validation");
+      return;
+    }
+    if (!projectId) return;
+    // A sign-in page is the one case where the *window* size matters least and
+    // the layout matters most, so it gets the ordinary desktop viewport.
+    openPageInContainerBrowser(projectId, safe, 1280, 720)
+      .then((result) => {
+        const push = useAppState.getState().pushToast;
+        if (result.error) {
+          push({ kind: "error", message: "The page didn’t open", detail: result.error });
+        } else {
+          push({
+            kind: "success",
+            message: "Opened in the container’s browser — see the project’s Browser tab",
+          });
+        }
+      })
+      .catch((e) =>
+        useAppState.getState().pushToast({
+          kind: "error",
+          message: "Could not open it in the container’s browser",
+          detail: String(e),
+        }),
+      );
+  }, [urlPrompt, projectId]);
+
   const handleScrollToBottom = useCallback(() => {
     const term = termRef.current;
     if (term) {
@@ -606,6 +650,7 @@ export default function TerminalView({ sessionId, active }: Props) {
           url={urlPrompt.url}
           label={urlPrompt.label}
           onOpen={handleOpenUrl}
+          onOpenInContainer={handleOpenUrlInContainer}
           onDismiss={() => setUrlPrompt(null)}
         />
       )}

app/src/components/terminal/UrlToast.tsx

@@ -6,6 +6,9 @@ interface Props {
   /** Heading above the URL. Says why the toast appeared. */
   label?: string;
   onOpen: () => void;
+  /** Open it in the container's own browser instead of the host's. Omitted when
+   *  the project has no browser to open it in. */
+  onOpenInContainer?: () => void;
   onDismiss: () => void;
 }
 
@@ -30,6 +33,7 @@ export default function UrlToast({
   url,
   label = "Long URL detected",
   onOpen,
+  onOpenInContainer,
   onDismiss,
 }: Props) {
   const origin = urlOrigin(url);
@@ -131,6 +135,30 @@ export default function UrlToast({
         Open
       </button>
 
+      {onOpenInContainer && (
+        // A sign-in completed in the *container's* browser lands its callback
+        // on the container's own loopback, which is where the tool waiting for
+        // it is listening — no host round trip, no auth bridge.
+        <button
+          onClick={onOpenInContainer}
+          title="Open in a browser inside the container, and watch it in the Browser tab"
+          style={{
+            padding: "4px 10px",
+            fontSize: 12,
+            fontWeight: 600,
+            color: "var(--text-primary)",
+            background: "transparent",
+            border: "1px solid var(--border-color)",
+            borderRadius: 4,
+            cursor: "pointer",
+            whiteSpace: "nowrap",
+            flexShrink: 0,
+          }}
+        >
+          In container
+        </button>
+      )}
+
       <button
         onClick={onDismiss}
         style={{

app/src/lib/tauri-commands.ts

@@ -1,5 +1,5 @@
 import { invoke } from "@tauri-apps/api/core";
-import type { Project, ProjectPath, ContainerInfo, SiblingContainer, AppSettings, UpdateInfo, ImageUpdateInfo, FileEntry, WebTerminalInfo, SttStatus, GatewayStatus, InstallOptions, ClaudeSession, ContainerCapabilities, ScheduledTask, ScheduledTaskInput, SchedulerNotification, AuthBridgeStatus, BrowserViewStatus, BrowserViewPopoutState, PlaywrightDetection, BrowserSetupOutcome, BrowserInstallTarget, ContainerStaleness, MigrationOptions, MigrationReport, MigrationState, ClearTokenOutcome, CaCertInfo } from "./types";
+import type { Project, ProjectPath, ContainerInfo, SiblingContainer, AppSettings, UpdateInfo, ImageUpdateInfo, FileEntry, WebTerminalInfo, SttStatus, GatewayStatus, InstallOptions, ClaudeSession, ContainerCapabilities, ScheduledTask, ScheduledTaskInput, SchedulerNotification, AuthBridgeStatus, BrowserViewStatus, BrowserViewPopoutState, BrowserPageState, PlaywrightDetection, BrowserSetupOutcome, BrowserInstallTarget, ContainerStaleness, MigrationOptions, MigrationReport, MigrationState, ClearTokenOutcome, CaCertInfo } from "./types";
 
 // Docker
 export const checkDocker = () => invoke<boolean>("check_docker");
@@ -222,6 +222,39 @@ export const closeBrowserViewPopout = (projectId: string) =>
  */
 export const getBrowserViewPopoutState = (projectId: string) =>
   invoke<BrowserViewPopoutState>("get_browser_view_popout_state", { projectId });
+/**
+ * Open a URL in a browser *inside* the container, published so the pane shows it.
+ *
+ * The same action serves an auth URL — the OAuth callback listener is in the
+ * container too, so the loop closes without the host — and a dev server on
+ * container loopback, which is how you watch a UI Claude is building. Only
+ * http/https; the backend rejects anything else.
+ */
+export const openPageInContainerBrowser = (
+  projectId: string,
+  url: string,
+  width: number,
+  height: number,
+) => invoke<BrowserPageState>("open_page_in_container_browser", { projectId, url, width, height });
+/** Resize that page. Real reflow, not a scaled screencast — see BrowserTab. */
+export const setContainerPageViewport = (projectId: string, width: number, height: number) =>
+  invoke<void>("set_container_page_viewport", { projectId, width, height });
+export const getContainerPageState = (projectId: string) =>
+  invoke<BrowserPageState>("get_container_page_state", { projectId });
+export const closeContainerPage = (projectId: string) =>
+  invoke<void>("close_container_page", { projectId });
+
+/**
+ * Make the page track the pop-out window's size as it is dragged.
+ *
+ * Only affects a page this app opened: a bound browser admits no second client,
+ * so one `@playwright/mcp` launched keeps the viewport it was given.
+ */
+export const setBrowserViewMatchWindow = (projectId: string, enabled: boolean) =>
+  invoke<void>("set_browser_view_match_window", { projectId, enabled });
+export const getBrowserViewMatchWindow = (projectId: string) =>
+  invoke<boolean>("get_browser_view_match_window", { projectId });
+
 /** Pin the pop-out above other windows — the point of popping it out at all. */
 export const setBrowserViewPopoutAlwaysOnTop = (projectId: string, onTop: boolean) =>
   invoke<void>("set_browser_view_popout_always_on_top", { projectId, onTop });

app/src/lib/types.ts

@@ -535,6 +535,23 @@ export interface BrowserViewPopoutState {
   always_on_top: boolean;
 }
 
+/** Mirrors Rust `page::Viewport` — CSS pixels, clamped backend-side. */
+export interface BrowserPageViewport {
+  width: number;
+  height: number;
+}
+
+/**
+ * Mirrors Rust `page::PageState`: what the container-side helper reports about
+ * the page Triple-C opened. `ready: false` with no error means there is none.
+ */
+export interface BrowserPageState {
+  ready: boolean;
+  url: string | null;
+  viewport: BrowserPageViewport | null;
+  error: string | null;
+}
+
 /**
  * Payload of the `browser-view-popout-changed` event: a `BrowserViewPopoutState`
  * plus the project it belongs to.