Just to provide feedback:
#2439 has not only the impact returning non-integer values.

I have a CSS transform:scale(2) in a root element with a div which has css width 200px.
jQuery 2.x returns 200 for .width() as it uses offsetWidth
jQuery 3.0 returns 400 for .width() as it uses getBoundingClientRect()

This is a breaking change which should be at least mentioned in the upgrade guide.