[Bug]: Desktop (macOS): an html_render page's fullscreen now takes the whole screen in a new Space instead of filling the T3 window #33709

Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/desktop

Steps to reproduce

1. In the desktop app on macOS, have an agent html_render a page with <button onclick="document.documentElement.requestFullscreen()">full</button>.
2. Click the button.

Expected behavior

As before #25835: the page fills the T3 window (the window has disableHtmlFullscreenWindowResize: true).

Actual behavior

The window goes into macOS native fullscreen in a new Space. Leaving it animates back; it feels like a glitch.

Impact

Major degradation or frequent failure

Version or commit

0.1.9-nightly.20261110.4452 (since #25835, c5b869f)

Environment

macOS, T3 Code desktop (Nightly)

Logs or stack traces

Cause: #25835 added a `fullscreenchange` listener in `apps/desktop/src/preload.ts` that sends `element !== null && element.tagName !== "WEBVIEW"` to `syncHtmlFullscreen`, which calls `window.setFullScreen(true)`. An html_render page lives in an `<iframe allow="fullscreen">` (BrowserDocumentFrame), so its fullscreen now counts as a chat video's.

Fix options, either works:
- keep iframe fullscreen inside the window as `<webview>` guests are: also skip `element.tagName === "IFRAME"` in the preload check;
- or a page message (like `t3/notifications/open-image`) that opens the page in the expand dialog.

Screenshots, recordings, or supporting files

No response

Workaround

The page skips requestFullscreen when the user agent has Electron/ and shows a larger inline view instead.