When browser.viewport is set and with the webdriver backend, sometimes parts of the app are unreachable

Describe the bug

I got this issue in the context of Webdriver + Firefox, where a button was in the viewport but not visible. In that case the click on the button succeeded but no action really appeared.

I can see several issues:

1. Chrome is launched fullscreen but Firefox is launched as a window, so the issue can happen much more easily in Firefox.
2. With playwright the application is "zoomed out" so that the viewport fits in the available space, but in webdriver the zoom level is always 100% (both in Chrome and Firefox). I don't know if that's a issue with webdriver or vitest or vitest's configuration of webdriver.
3. Part of the issue, I think, is also that vitest's interface can not be scrolled.
4. In Chrome, when running the test after reducing the window size, I get an error but in Firefox there's no error making the issue silent -- This is probably rather a bug in Firefox and I'll report it there too (bug 1994715).

Reproduction

You can find a testcase in this repository: https://github.com/julienw/vitest-browser-mode-viewport-click
The main branch uses webdriver while the playwright branch uses playwright so that it's easy to compare.

System Info

System:
    OS: Linux 6.7 Debian GNU/Linux 12 (bookworm) 12 (bookworm)
    CPU: (16) x64 11th Gen Intel(R) Core(TM) i9-11900H @ 2.50GHz
    Memory: 29.55 GB / 62.54 GB
    Container: Yes
    Shell: 5.2.15 - /bin/bash
  Binaries:
    Node: 24.8.0 - /home/julien/.nvm/versions/node/v24.8.0/bin/node
    Yarn: 1.22.22 - /usr/bin/yarn
    npm: 11.6.0 - /home/julien/.nvm/versions/node/v24.8.0/bin/npm
    pnpm: 10.9.0 - /home/julien/.local/share/pnpm/pnpm
    bun: 1.0.29 - /home/julien/.bun/bin/bun
    Deno: 2.0.0 - /home/julien/.cargo/bin/deno
    Watchman: 4.9.0 - /usr/bin/watchman
  Browsers:
    Chrome: 139.0.7258.127
    Chromium: 139.0.7258.127
    Firefox: 128.13.0esr
    Firefox Developer Edition: 128.13.0esr
  npmPackages:
    @vitest/browser: ^3.2.4 => 3.2.4 
    playwright: ^1.49.1 => 1.56.0 
    vite: ^7.1.10 => 7.1.10 
    vitest: ^3.2.4 => 3.2.4 
    webdriverio: ^9.20.0 => 9.20.0

Used Package Manager

npm

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

Start with the linked vitest-browser-mode-viewport-click reproduction and compare its main webdriver branch with the playwright branch. Investigate the reported browser viewport, window size, zoom, and scrolling behavior in the reproduction; done means the webdriver case no longer allows clicks on unreachable or invisible parts of the application without a visible result.