Bugzilla

STR:

According to mozregression bug 1255108 and bug 1255106 introduced this png display artefact:

According to mozregression bug 1255108 and bug 1255106 introduced this png display artefact as shown in the screenshot.

STR:
1. Install brief from https://addons.mozilla.org/en-US/firefox/addon/brief/
2. Go to any rss feed like https://planet.mozilla.org/
3. In the page action bar (in awesome bar) click on the RSS feed to subscribe
4. In browser action bar click on brief RSS icon
5. In Brief click on "all items" 
6. Then switch to "View headlines" in brief (which is on the top right). Looks like a hamburger menu

ER:
There should be no graphic glitches

AR:
7. On the right the arrow png icon has an artefact recently.



I tried to create a minimal working example at https://codepen.io/nachtigall/full/zQQWYB but was no really able to get all the relevant html/css in it to reproduce it, but maybe it helps as a skeleton... 

fwiw, I run this:
```
$ mozregression --bad=2019-06-05 --good=2017-05-20 --addon brief-2.6.0b2-fx.xpi --arg="https://planet.mozilla.org/"
...
86:48.16 INFO: Last good revision: ca4f3b736dd6cbe6e1b195a4800b009ea55bf91d
86:48.16 INFO: First bad revision: 44fa715640dd2a4d06014e6d5fdda63c52707188
86:48.16 INFO: Pushlog:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=ca4f3b736dd6cbe6e1b195a4800b009ea55bf91d&tochange=44fa715640dd2a4d06014e6d5fdda63c52707188

```