I have no second monitor and can't test it, so I've just slapped some random code hoping it will work.
A Borderless fullscreen option will be added to the Settings in build 205, it has to be tested - keeping the issue open until someone tests this new setting.

I have previously two monitors and windows (10) looks like to have kept this info despite one is no longer there and on top & left sides of the screen, mouse pointer is kept there and cannot move further away up or left.
But on right & bottom sides, according to my previous dual screen settings maybe, mouse pointer is able to go further, I don't see it at all but it is not kept higher & lefter than these screen boundaries.

I guess what you've add is not enough.

Could it be possible to force mouse pointer inside screen boundaries ?

I can not debug your case with a single monitor, and trying to fix Windows' bug by the means of the game doesn't sound healthy.
Can't do anything about that at this moment, keeping the thread open for a feedback from other players who have multiple monitors