Whether the document should be immersive.
fullscreen also asks the browser for fullscreen — best-effort, a refusal
never propagates, because a caller that entered immersive mode has still
entered it. See the note on ownership: an exitFullscreen only happens when
this module was the one that entered.
⚠️ The shape is inlined rather than a named exported interface: nothing outside needed to name it, and an exported type nobody imports is indistinguishable from one left behind after a deletion.
Turns the immersive UI mode on or off.
The mode strips the chrome the kernel owns — theme bars, filter entry point, right panel, proximity bar — so a caller that needs the whole surface for one task can have it without knowing which controls exist.