I'm not a 100% sure but I guess I had to press right shift to unlock the mouse and being able to shoot with LMB, after keep pressing space. When not pressing space (eg behind the crate) the mouse was freely moveable. By pressing space my sight moved up above the crate, but the crosshair/ mouse would be locked in the last (screen) position, until I pressed shift once, or was it "-", left to the shift key... this was Brave (based on chromium) browser on win10.