actually for example on the previous outlook of facebook this was a big problem, as notices were under the return from full screen button. try pushing it, and the return button would pop up. Doesn't affect it anymore, as new layout on facebook. But it still can be a problem.
There should be a option to disable it imho, or just have it somehow that double tab bottom right corner for it to pop up.

This is also really annoying when using rdesktop, which uses the same fullscreen mode thingy.