Making Steam Input the default backend silently broke input forwarding. Merely
initializing Steam Input activates Steam's in-process XInput interception, which
hides controllers from XInputGetState unless they're bound to our action set for
the running appid. With no such binding (the normal case for a donor appid) Steam
Input reports zero controllers AND XInput now sees nothing -> no input at all.
Reproduced with coop_steam_input_probe: without Steam, XInput slot 0 is seen;
with Steam Input initialized, 0 Steam controllers and the XInput fallback goes
empty.
Default to XInput (RPT delivers guest pads there and it works) and make Steam
Input an opt-in Controllers-panel toggle that switches the backend at runtime;
turning it off restores XInput. All 6 tests pass.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
BlackMark
914dba4d25