I saw this in a webpage, and was breaking the arrows for their dropdown menus (the style was used in a pseudo-element).

Here's a screenshot of the rendering I see, in case it's graphics-card dependent. This is a skylake integrated GPU on Linux. Also, the extra border doesn't seem to depend on the size of the <div>. 12px is the size I saw it in the page, but it works with other sizes too.

A duplicate of bug 1399485 and/or bug 1393907?

This got fixed. :) mozregression --find-fix --bad 2018-01-15 --good 2018-01-17 --pref gfx.webrender.all:true startup.homepage_welcome_url:"https://bug1427654.bmoattachments.org/attachment.cgi?id=8939433" > 3:08.87 INFO: First good revision: 5a130721f8ea9793e201453caf4bc56d5b3ae53f > 3:08.87 INFO: Last bad revision: b2cb61e83ac50115a28f04aaa8a32d4db90aad23 > 3:08.87 INFO: Pushlog: > https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=b2cb61e83ac50115a28f04aaa8a32d4db90aad23&tochange=5a130721f8ea9793e201453caf4bc56d5b3ae53f > > 3:11.21 INFO: ************* Switching to autoland by process of elimination (no branch detected in commit message) > 3:11.21 INFO: Getting autoland builds between 2366aeabd64c6ea5c7723b8f6bbcf3d5c0c85d46 and 5a130721f8ea9793e201453caf4bc56d5b3ae53f > 3:11.72 ERROR: The url u'https://hg.mozilla.org/integration/autoland/json-pushes?fromchange=5a130721f8ea9793e201453caf4bc56d5b3ae53f&tochange=2366aeabd64c6ea5c7723b8f6bbcf3d5c0c85d46' contains no pushlog. Maybe use another range ? I think my bug 1429614 was more like a duplicate of this one, but I can't assess that. Sorry. Both rotate something.