Discovered while working on #2991

Safari and IE11 are the two browsers we support which do not have ResizeObserver. For those, we fall back to a MutationObserver watching for all changes. While that catches when mutation cause a different size, it incorrectly:

• misses size changes caused by a window resize
• catches content changes that do not affect size
• misses size changes caused by changes in a document's CSS

I don't think we should try addressing the third one, but wanted to include it for completeness. The other two should be fixed by changing the fallback to:

• listen for resize events
• confirm that a newly computed size is, in fact, different from the previously known one