[garbear]

The toggle button + second button is the best option.

This is not only to show the OSD or the game in full screen. With tooggle you can remap all your pad with system functions. In retroarch and RetroPie (that uses RetroArch) i can click:

Select (toggle) + Start: Exit

But you can have combinations to: Save state (in current slot), load state (in current slot), screenshot, menu, fast forward, change savestate slot... (any option that can bin binded).

So with select + r1/l1 i can do a fast save state / load sate, and with select + second analog left/right i can change the savestate slot.,
WIth select + one button i can take a screenshot.

The problem with combos is that not all controllers have the same buttons. Genesis controllers are missing a Select key. Some controllers are missing shoulder buttons or triggers. The gamecube controller has two triggers but only 1 shoulder button, for example. This means we can't show button names in the GUI, and documentation will always need a long explanation of logical vs. physical controller mappings. etc.

In RetroArch, if select is used as a combo button, how do you send a select press to the game? If you press Select + L1, does the select press also get sent to the game?