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.

The problem is intermittent but occurs frequently. It may be a JavaScript race condition (order of operations) because the bug disappears when we turn on debugging.

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.