Below 782px viewport width we lose the "W button" from the top left corner. This makes it almost impossible to access the browsing sidebar on mobile and tablet in the Site Editor. Almost, because we can still access it from the Document Settings dropdown.

This also makes it impossible to leave the block editor without turning off fullscreen mode.

After all, we can rely on the admin bar but why do we have the button on the desktop then? I think it would make sense to keep the W button visible all the time to avoid confusion.

Editing a page:

Site editor: