Ensure onHeightChanged is called after DOM change

[jryans]

onHeightChanged is used to notify the scroll panel of dynamic height changes in the timeline.

In a few spots, it looks like we might be calling it too early before the DOM has actually been updated:

• MImageBody
• MVideoBody

This could be a root cause of some scrolling confusion.

[andybalaam]

@jryans is this still relevant? If we're not actively working on it, I suggest we should add more detail about the problem we think this causes.

[jryans]

@andybalaam Hmm, I believe the context is some components are calling onHeightChanged before the height of the actual elements have changed. For example, MVideoBody calls it just after setting state to the content URL, but setState is async, so it's unlikely that the video URL has been rendered to the DOM, and thus the video element may still have the wrong height when onHeightChanged.

In any event, I believe this was mainly a theoretical concern I noticed while browsing the code for some other reason. It may be more efficient to close and wait for someone to report the visual issue of scroll jumps, which is how this could manifest if it truly is a problem.

[MadLittleMods]

Related to #23539