Like the title says, and to be clear this only occurs in Source Unpack v2, in both HL2 and Portal. This doesn’t happen in the currently updated versions on Steam.
I know the readme says to email Bitrain, but if other people can help, or if a solution is found here and the thread helps others, I think this is the better way.
So moving the mouse in any way in-game causes the framerate to drop to nothing, clicking doesn’t affect it, and using the mouse outside the window while alt-tabbed is still fine, but moving the cursor back into the game window not only lags the game but brings the mouse movement to a dead stop until the cursor is no longer moving.
I can’t find anything in the console to fix it, at least not mouse related, and I can’t find any relevant differences between it and the Steam copy.
Vsync doesn’t help, and whether it’s properly in-game or even in the opening menus it still happens.
If any more information is needed I can provide, or record footage of it.