ZDoom

...after using the mouse.

I have to use the keyboard to turn left/right or something for it to come back, but after I use the mouse again it returns. This is different from Raze where the interpolation for quick turning doesn't get lost after using the mouse, which makes me think this is a GZDoom bug.

I don't think this is fixable without redoing the entire code involved here. The mouse movement totally bypasses the input code and will screw up any movement induced through the ticcmd. This was what made us decide for Raze to not copy this code but handling it differently.