In WikiEditor, when I click a dropdown such as "Advanced / Heading" in the edit bar, sometimes the dropdown menu does not appear. It looks like the dropdown menu is rendering behind the edit box, i.e., the Z-index of the menu is incorrect. This is on a Macintosh (macOS Sierra 10.12.1) using Google Chrome (56.0.2924.87 64-bit). The problem does not occur in Chrome on our Microsoft Windows computers.
We have reproduced this issue on two different Macs. It occurs on English Wikipedia today (MediaWiki 1.30.0-wmf.17 (259e37a) 21:23, 7 September 2017) and on our own MediaWiki 1.28.0 server.
I am attaching screenshots of the Heading dropdown when closed (closed.png), when open normally (open-normal.png), and when open as the bug occurs (open-with-bug.png). Notice in the third screenshot (with the red box), the first few pixel-rows of the menu appear -- you can tell because the area immediately below "Heading" is white rather than blue.