When I adjust the scale of a layer (using the transform handles in comp view), I can hold shift to evenly scale on both axis. But it doesn't actually maintain the proportions, it just copies one axis to the other. So any flipping or aspect ratio changes are lost.

I'd like to be able to scale objects proportionally to the existing size, so that flipping/stretching is not lost.

This is actually already possible if I resize by sliding the scale value in the layer's properties (with constrain proportions enabled), but scaling from the comp view is much faster and more intuitive.