Left Mouse Click Causes Windowed Mode

Have you checked your control settings in the game and your mouse settings in Windows to make sure you haven't got the LMB set up to do anything... odd?

Are you using some sort of gaming mouse such as a razor? Sometimes they can cause issues with this game.

No, just standard hardware. I found that it doesn't seem to happen if I use Fullscreen Windowed mode or whatever it's called but I don't know if that is comparable to playing in Fullscreen mode.

I'm actually playing on a desktop using an NVIDIA GeForce GTX 960 which should not have any graphics issue with this game, I hope. I have no problem with keeping it in the mode it's in now unless it's degrading performance by not being in fullscreen mode. I just don't understand what would cause the problem I'm having since the game and hardware are fairly current.

Ahh, but you forget, the game is also SUPER buggy. 😉 
I can't think of any other suggestions for causes right now, but at least you have a temporary solution in windowed mode (some people even seem to prefer playing it that way). Perhaps a dxdiag might provide more hints?

i'm going to run some FRAPS tonight to see why kind of FPS I'm getting in this mode. It doesn't seem like the performance is degraded (at least to my eye) so I think I'm just going to let it ride instead of dragging this issue out.

You ever figure out the problem of going into windowed mode when you left click?

@poetic332 Playing in fullscreen windowed mode seemed to fix the issue for the OP, since that worked I think they decided to just leave it rather than try to dig in to what was causing the issue in the first place. If you're having the same issue, some information about your system (such as a dxdiag) and any differences between your problem and that experienced by the OP might come in handy. 😉

Sorry. I did decide to just leave it in the Full Screen Windowed Mode since it didn't seem to cause a performance decrease. I will mark this as solved.