Steps to reproduce:
1. Open user's attached project
2. Select "Director" gameobject in the Hierarchy
3. Select Timeline window
4. Slightly drag the bottom of the Timeline
5. Try grabbing the side scrollbar

Expected result: Side scroll bar works correctly after resizing the Timeline
Actual result: Dragging the bottom of the Timeline causes several incorrect behaviours

Reproduced with: 2017.1.1f1, 2017.2.0b11, 2017.3.0a7

Note: Side scrollbar not working is only one of the incorrect behaviours. Another one is that moving the mouse cursor to the left/right edge of the Timeline window makes it scroll horizontally.