How to reproduce:
1. Open the attached project
2. Select Assets/Sprites sprite sheet
3. In the Inspector window, click several times on one of the drop-down menus

Expected result: There is no Sprite Sheet grid artifact on the Preview window.
Actual result: Sprite Sheet grid artifact appears on the Preview window.

Reproducible with - 2018.1.0a6, 2018.1.7f1, 2018.2.0b11, 2018.3.0a3
Not reproducible with - 2017.1.4p2, 2017.2.3p1, 2017.4.6f1, 2018.1.0a5

Note: The issue is reproducible on the Scene window as well.

Fixed in 2019.1.0a2