Window tabs are glitching on Asset store window resizing.

To repro:
1. Create a layout with two pane, upper pane with Asset Store and lower pan with anything else (i.e. Console window)
2. Horizontally resize it.

Actual result: Tabs view is glitching (see attached gif). Error in log:

GUI Window tried to begin rendering while something else had not finished rendering! Either you have a recursive OnGUI rendering, or the previous OnGUI did not clean up properly.

Expected result: No visual artifacts appear in Editor.

MacOS 10.13.
Reproduce in 2017.2.0f3 (46dda1414e51)