so I was having issues with get_mouse_coords() and just wanted to share my solution for this. even though I had my view, viewport, camera, and crt settings all set to a window width of 540, for some reason, window_get_width() would return a value of 514. so I replaced the code in get_mouse_coords() to use the actual window dimensions. this isn't pixel perfect, but it is very close & functional:
var u = display_mouse_get_x() / window_get_width();
var v = display_mouse_get_y() / window_get_height();