If you set an objects width or height to a negative number to mirror/flip it, the corner controlls no longer function, so once flipped, the objects dimensions can only be adjusted by typing numbers. :( Why not just let the user drag it into negative numbers? If its somehow a problem, what if they have to hold a key while dragging in order to "go negative" That would be a decent compromise. Lastly, an Ideal feature would be that pressing "x" would automatically horizontally flip the object(s) you have selected and pressing "y" would vertically flip.
