I have a Pro 2 too and switched from Xinput to Dinput at the back of the controller and my Steam Deck now identifies the back buttons like L4/R4 (but with the name PL/PR) and I can map it to anything I want
I have a Pro 2 too and switched from Xinput to Dinput at the back of the controller and my Steam Deck now identifies the back buttons like L4/R4 (but with the name PL/PR) and I can map it to anything I want