Same issue here. I have to right click --> Fullscreen for it to work. Super annoying.

I've had the same issue. The mouse position, it seems, has an x-offset in relation to the actual cursor position. Similarly to Guardian2300, all buttons are placed in the correct positions but interacting with them requires the mouse be to the left of the button. This only happens in full screen mode.
If the cursor is all the way to the left of the screen, the positioning is fine. However, as the cursor moves farther to the right, the offset gets larger. My game, which is 2D, uses a drag & drop mechanic so the difference is easy to spot. I recently ran a playtest and this happened on every machine I ran it on, forcing me to limit play to Windowed mode.

It happens with the last version as well 5.1.1p1

I've had this problem as well. The buttons would be in the correct place but when you want to click them, for me I have to get to the left of the button and click for it to work.

Same issue here. All mouse input is incorrectly shifted.
1024x768 is aspect 1.33, which is less than 16x9 monitor with aspect 1.77. Screen is stretched horizontally, but all mouse input is shifted (not only UI items) as if the screen had 1.33 aspect and started in bottom left corner.
If run with -force-d3d9 then game has pillars on sides, but input is correct.
Input is also correct if built for (not recommended) Exclusive mode (either for DX9 or DX11) - it is correct even if screen is stretched.