Bug with headers in stock sample/demo

[jimfb]

• I have searched existing issues and this is not a duplicate

Issues and Steps to Reproduce

1. Star the sample app.
2. Scroll down until the article on Gaston Marichal (under 1830) is half-way off screen.
3. Scroll back up a little bit, so the date changes to 1810.
4. Observe the 1830 header is not detaching from the fixed header and is not becoming part of the document.

Expected Behavior

The 1830 header should pop back into the scrollable surface. Currently the header does not pop into the document, which results in a strange/glitchy header.

Link to Code

Standard litho sample/demo as of today. Video: https://www.youtube.com/watch?v=KEbN1-iriSk&feature=youtu.be

[jimfb]

Eeh, looks like a duplicate of #106, although for a different demo. I'll close it for now. Feel free to reopen if you'd like to track the two demos separately.