15b4: Bug with timeline thumbnails

Thumbnails won't render on the timeline if these steps are followed:

1. Open a timeline and play
2. Zoom out (alt-mousewheel down) so the timeline collapses to a small sliver
3. Zoom back in (alt-wheel up) and thumbnails aren't shown. This much might be intentional.
4. While playing, grab the playhead and move to a different location.
5. Now stop. Timeline thumbnails will not display until zoom level is adjusted again. However the behaviour is different if you skip step 4 (in which case thumbnails appear once transport is stopped).

(Not sure if a regression, I just happened to notice it now.)

[Peter Chamberlain]

Which OS Jason?
Which GPU?

Sorry Peter, I guess I wrongly assumed this must have been a basic logic thing independent of platform.

Windows 10 x64 (Version 1709 Build 16299.431) and NVIDIA GeForce 1080 Ti with driver 391.35. 8700K with 64GB.