RFP when switching windows to fullscreen without letterboxing
At this moment we show the maximization warning after user is back from fullscreen with F11.
The warning being shown is an implementation artifact that's always been there: notification bars cannot be shown in fullscreen mode: we either need to change the widget implementation or to keep the existing behavior.
Another option would be prompting for per-site permission before proceeding with fullscreen, which is something which we've been also considering for content-triggered fullscreen (not explicitly initiated by user interaction with the browser UI) as an alternative to letterboxing.