Steps to reproduce:
1. Open any project
2. Open animation window
3. Create new animation
4. Reduce the size of the window so that the slider overlays the left panel
5. Returning the window to normal size has the slider glitched over the panel up until a certain size

Expected: As the window is reduced, the panel is occluded by the timeline, when restoring the window size, the same should happen but backwards
Actual: When restoring the window size the slider is glitched on top of where the panel was supposed to be.

Reproduced on: 2018.1.0a2, 2017.3.0b6, 2017.2.0p1, 2017.1.2p2, 5.6.4p1,

Note: Also, resizing the left panel when window is at the critical small size yield weird results. Check repro video for clarification