Checks the proportional scaling of a selected object.
If the XY scaling difference is 0%, the object is scaled proportionally, i.e. the height-to-width ratio is maintained. The object is enlarged or reduced both horizontally (X) and vertically (Y) with the same percentage.
In some cases, a small XY scaling difference is allowed, for example in line art (e.g. a rectangle in one color); in images this may cause distortions.