I’ve had this problem for a month or so, but never figured out how it happens until now.
It happens whenever I press the windows key(start menu opens) at the same time that the game launches to fullscreen.
To reproduce launch Siralim 2 and before it opens rapidly press the windows key. The mouse cursor will display in fullscreen mode and all user interactions will be with what was behind the window. So if Firefox was opened than all user input will be directed to Firefox and not Siralim 2.
The game even displays over the lock screen(Win+L) and the lock screen only shows up clicking or typing. User interaction upon unlocking is directed to the game.
To fix this Task manager has to be opened and alt-tabbed(I have it to display above all windows). Once a window is above the game the issue resolves itself.
I don’t know if this is a bug with Siralim 2, Steam, or Windows 10.