As much as I continue to try, the extension doesn't seem to work exactly like I want it. I was wondering if there was a small guide for use.
Constrains the mouse cursor to a rectangle at system level · By
There's a demo project (GMZ) included.
Usually you do
if (window_has_focus()) {
display_mouse_lock(window_get_x(), window_get_y(), window_get_width(), window_get_height());
} else display_mouse_unlock();
to lock the mouse to window rectangle (and unlock when the game doesn't have focus) and that's it really