API-triggered fullscreen after F11 causes letterboxing to crop the page

!480 (merged) accidentally "fixed" (pending agreement on whether fullscreen should be letterboxed or gated being a permission prompt) #32713 but introduced an interesting regression: if a page obtains fullscreen through the requestFullscreen() Web API while the browser is already maximized (e.g. via F11 key), letterboxing persists as it is but the page gains the full screen dimensions nevertheless, possibly causing it to be cropped.

Steps to reproduce:

1. Open a Youtube video page in Tor Browser 12.5a1
2. Maximize the window (hit the F11 key)
3. Click the fullscreen button in the video player UI
4. Watch the page being cropped by letterboxing.