GAUD-10313 - Fix jitter if pulling the panel backwards when collapsed

[svanherk]

This may be a Windows-only thing, as I can only make it happen on certain screen sizes/monitors. But if you try to pull the divider backwards while collapsed, you get this jittering:

You also get the big Windows scrollbar appearing, and then animating away.

It's really complicated to not have the scrollbar appear at all, since we need it there as we start dragging open.

The easiest way to fix this is to turn off animating until you're actually opening the panel. I think this looks better, but it does mean you "snap back" to closed if you didn't pull the panel far enough to open:

And here it is with a scrollbar:

If this solution is not acceptable, I can go back to the drawing board, but this is the cleanest I've found so far.

[github-actions]

Thanks for the PR! 🎉

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URL	https://live.d2l.dev/prs/BrightspaceUI/core/pr-7426/

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[svanherk]

Ignore this for now, still trying to find something better.