Program that don't use windows own window decorations are detected as fullscreen.

[blaberry]

For example, the main Steam window is detected as fullscreen if it's maximised, as the window controls are inside of the application drawing area.

[blaberry]

More specifically, this happens if steam is maximised on the screen that does not have the taskbar.

Hacky solution: blacklist steam from causing the mouse lock. Doesn't help the general cause however.
Good solution: Not sure yet. Something along the lines of inspecting the window in question, maybe asking whether it contains it's own window controls. But then the question is how many applications report this incorrectly.