Position of popup may not be correct when first opened, when closed and reopened it's generally right

[pbagnall]

Seems there's an issue with measuring the popup's size in the first instance.

[pbagnall]

The problem is that the width of the popup depends on where it's positioned. So for each test we need to position, measure, and then decide if it fits. Assuming the size will be constant through all tests doesn't work. This might mean we take a few frames to try out various positions.

[pbagnall]

Moving the popup to the end of the DOM, allow it to easily sit above other content, rather than getting clipped or causing scrolling. Positioning of the popup now works in that scenario.

It still doesn't work when the popup is elsewhere in the DOM, specifically if it's a descendent of a positioned element. At the moment, it's not adjusting coordinates for the context.

[pbagnall]

This is now mostly fixed. Need to support arbitrary nesting of scrollable containing elements, and test.