The mouse always starts in the bottom right hand corner

[eddebaby]

It seems when a screen is initialised (that uses the mouse cursor) the mouse cursor's position is initialised to the bottom right hand corner of the screen. This is with the standard input mode, I haven't yet tried with altinput.

This happens on the menu screen, when loading in to the game, and when switching resolutions in-game. Probably happens when the window size/scale is set.

[Loobinex]

I've seen it happen, but for me this is not the case. Are you sure you've got your movies, menu and ingame resolutions all to the same value?

[AdamPlenty]

This happens to me if I start the game with the -level parameter to load a specific level.

[Loobinex]

Not for me, starts dead center. Could you try to set your game resolution to your native desktop resolution adam and see if it still happens?

[AdamPlenty]

I set everything to 1920x1080, but the cursor still started at the bottom right when I use the -level parameter.

[Loobinex]

I think it also depends on where your cursor is when you start the game.

[eddebaby]

I have different resolutions for in-game versus menu/movie. I'll investigate with same resolutions etc.

Edit 1:
If I set all resolutions to 640x480, this error doesn't happen on menus, but when I enter the game, and then touch the mouse, the cursor is flung down and to the right by about 25% of the screen.

Edit 2:
With all resolutions set to 1080p, again the error is gone for menus, but when touching the mouse after being in-game, the mouse is flung right to the bottom right corner of the screen.

[Loobinex]

For me it really seems to be linked to where I have my mouse when I start the game. Only if I have my mouse at the bottom left of my screen (like, where the ok button on the windows RUN field is) do I get the behavior you describe.
If my mouse is top left, top right or bottom right of the screen, the cursor stays in the center of the screen without jumping.