Improve window resizing - when widgets added/removed

[jarvisteach]

As demonstrable in issue #279 on Mac.

When widgets are removed from a window, then new ones are added. The new widgets will scale to fit the original window, and not rescale to match the new window size.

In the above scenario - the original window is bigger, so when it shrinks, the buttons are missing their bottom edge.

[mpmc]

Also, constantly moving the window during a pending action causes similar behaviour.

I've attached a video showing the issue - I had to zip it as I couldn't upload webm format to github.

example.webm.zip