Popup positioning improvements

[toasted-nutbread]

This change does some refactoring to popup positioning and updates how the popup is positioned for source content which spans multiple DOMRects.

The updated algorithm is now:

• Iterate over each sourceRect, calculating the popup rect for each rect rather than the full bounding rect.
• If the popup rect doesn't overlap with any other sourceRect and if it has the best size, use it as the optimal position.
• If no optimal position is found, calculate the bounding rect of all sourceRects and return that position. (This is the same as the old behaviour.)

Resolves #2124.