Alt-tabbing the game in windowed mode alternatively reveals and hides the UI, due to the [Tab] button being used to hide the UI. Have not tried in other windowed / full-screen settings, but would expect the same behaviour.
Steps to reproduce:
Start a game, alt-tab out of it and back into it.
Nothing happens: game is able to distinguish between Alt-Tab & Tab.
Versions and Mods:
r134, no mods.