R1:7729c51cc4f2

Fix an issue where scrolling down, then up, then down fails to show changeset header in Differential

Summary: Ref T13151. See PHI616. There's a bug where the current banner changeset isn't cleared correctly when we hide the banner.

Test Plan:
- View revision with several changesets.
- Scroll down slowly through first changeset until banner appears.
- Scroll up until banner disappears.
- Scroll back down.
- Before: banner fails to reappear (code still thinks it's visible and we don't want to…