Steps to reproduce:
1. Open attached 'TimelineTrackIssue.zip' Unity project
2. Go to Window->Timeline Editor
3. Quit Unity
4. Reopen project
5. Click on ‘Cube’ object in the Hierarchy tab
6. Look at the Timeline tab

Expected results:
Timeline tracks are visible
Actual results:
Timeline tracks are not visible

Reproduced on: MacOS 10.12, MacOS 10.12.5
Reproduced in: 2017.3.0a2, 2017.2.0b5, 2017.2.0b3, 2017.2.0b2
Didn’t reproduce in: 2017.2.0b1, 2017.2.0a3, 2017.1.0p2, 2017.1.0f, 2017.1.0b1
Regression introduced in: 2017.2.0b2

Note: Closing and reopening the Timeline tab makes the tracks visible again