In full screen mode, depending on the skin you're using, usually when moving your mousepointer to the bottom of the screen a contol-panel will appear.
I assume you mean that you are in full screen mode? If so (and unless you redefined the shortcut keys) [Esc] should bring you back to Window-mode. Also pressing [H] (default definition) should cause the control-panel to pop-up. (Also through rightclick > Show/hide main window)
In case you do mean On-top mode after all:
Always on top -switch is [Ctrl]+[A] (default definition) and
Always on top overlaymode is [Shift]+[A] (default definition)
A further option in this connection is available under
rightclick > Options > Always on top control window.
You can change shortcutkeys under:
(rightclick >) Options > Preferences > Key definitions & WinLIRC
(Note keys for windowed and full screen mode are defined separately!)
(P.S.: Under Options > Preferences > Video [General] you can find the option "Always start in full screen mode"
and under > Preferences > General there are the options "Show main window
(=control-panel) on mouse move" and "Show main window on single click" as well as "Auto hide main window"
(and others))