Some websites have pop ups that hide the navigation menus, a good example of this would be instagram popping up a login menu when you aren’t signed in. For some reason this is allowed to hide the navigation menus, I really don’t see a positive use case for this ability anywhere, literally being prevented from leaving sites is completely unacceptable.
Seconding this - to provide more context, Brave’s normal behavior is to hide navigation when you scroll down, and then the navigation reappears when you scroll up. But if you scroll down and then the page disables scrolling, you cannot scroll back up to get the navigation and you must close the app to leave the page. A potential fix would be to allow scrolling up just to get the navigation back without scrolling the page, even if the page has scrolling disabled.
Not sure if this is exactly what OP is referring to but it’s what I’ve experienced and came here to report.