For some reason the window_command_run function won't work if inside a mouse_check if statement. 

for example, this doesn't work even in the example project:

if(mouse_check_button_pressed(mb_left)) window_command_run(window_command_maximize);

Windows will not let you perform some operations while a mouse is being held on a window as this would cause a click through to a window behind it - you’ll need to wait until the mouse release event.