STR:

1. Visit the attached test case
2. Hover the Wikipedia favicon with your mouse

ER:

The transparent corners of the favicon should remain transparent.

AR:

The corners seem to darken.

The display density of your monitor may affect this bug. At a 1.25 scale, I was able to see the issue on both my Retina MBP display and on my lower resolution external monitor.

On a whim, I decided to see if this was a regression or not. I couldn't go back too many versions on macOS before hitting builds that just wouldn't run, but on Windows, I was able to go wayyyy back, and found that back at the start of 2016, this issue didn't appear to exist.

According to mozregression, the problem was introduced somewhere in this stack of patches: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=a551f534773cf2d6933f78ce7d82a7a33a99643e&tochange=c724ae8bb7b867b6bc65efa1049c0322457ff4b0

At least on my Windows machine (which I connect to over RDP), it looks like this may have been introduced by bug 1007702, which enabled Skia on unaccelerated windows. I tested that by running the "bad" build from my mozregression with the Skia pref flip reverted:

./mach mozregression --launch 2016-08-27 --pref "gfx.content.azure.backends:direct2d1.1,cairo"

Hmm, I think perhaps we weren't rendering this particular image correctly until bug 1404366 (dec 2017).

Set release status flags based on info from the regressing bug 1007702