I'm using MPC and I just redid my computer and now all of a sudden when I manually resize the player's window i.e. left mouse button on the windows frame and then pulling/pushing the window just stretches or contracts.
It used to resize proportionally, i.e that if I pulled horizontally it would automatically also adjust vertically to make sure the image remained in it's original AR.

Does anybody know how to fix this and get it back to keeping the AR correctly?