The tile indicator (small triangle tick between two clips when they are perfectly tiled) gets stuck in the UI when zooming the timeline.

repro:
- create a Timeline with several tiled clips
- autofit ("a") the Timeline
- use the mouse scroll to zoom into the timeline quickly...
bug: some of the tile indicators get stuck in the UI and no longer move until you zoom out. When you zoom out, these indicator take their apropriate place on the timeline.
(see attached gif)
expected: Tile indicators are always and only between two tiled clips.

this is a regression.
repro in...
Version 2019.1.5f1 (6f1140cf2297)
Branch: 2019.1/staging

NOT repro in...
2018.4.1f1