ScrollBar breaks when ScrollViewer content changes

[TomEdwardsEnscape]

Describe the bug
ScrollBar breaks when switching between tabs in the Control Gallery.

I spent some time trying to debug how I had broken this with the changes in #10803, before realising that the same bug also appears on master. If the fix for this isn't going to be simple, it may be better to wait until that PR completes to start on it.

To Reproduce

1. Start the Control Gallery
2. Switch to another tab
3. Switch back to the first tab
4. Scroll the tab content

If you scroll with the mouse wheel or by clicking the up/down arrows, the scroll bar doesn't move. If you scroll by dragging the scroll bar, it still doesn't move and while the viewer does scroll, the "scroll zone" is very small, resulting in much faster movement than expected.

Expected behavior
No change to scroll behaviour after switching back to the tab.

Desktop (please complete the following information):

• OS: Windows
• Version: master

[TomEdwardsEnscape]

Things have improved a little, and the very fast scrolling no longer occurs. But the scroll bar is still locked in place.

[TomEdwardsEnscape]

Can no longer reproduce.