Theme screenshots flicker on initial hover with fresh cache in Firefox.

[JoeFusco]

With a freshly cleared cache, theme screenshot images appear to jump/flicker/flash on the initial hover. This behavior is not intended and can be easily solved by using the will-change property to force hardware acceleration which fixes the initial paint. See: ​https://developer.mozilla.org/en-US/docs/Web/CSS/will-change

Due to how fast the flicker is, you will only see twentyfifteen and twentyfourteen theme screenshots flicker in the following screencast although all 6 screenshots flickered initially.

screencast: ​http://quick.as/9yj2sooqn

Steps to reproduce:

1. Open latest Firefox
2. Go to src.wordpress-develop.dev/wp-admin/themes.php
3. Refresh the page and bypass the browser's cache. (Mac cmd + shift + R)
4. Hover theme screenshots.

[JoeFusco]

See attached solution.

[afercia]

Thanks @JoeFusco and welcome to Trac! Seems something specific to OSX and webkit browsers, right? Couldn't reproduce on Windows + Chrome. Same for #35787.

[JoeFusco]

@afercia Thanks! That is correct. I was not able to reproduce this on Windows 10 Chrome, same goes for #35787 in Firefox. Both issues seem exclusive to OSX (10.11.3). Sorry forgot to include OS on this one.

[ocean90]

I can't reproduce this issue in Firefox 44.0.2 on OS X 10.11.3 with a Retina screen. Do you have any other plugins installed?

[JoeFusco]

No plugins or extensions. Turns out I was using 46.0a2 which is not stable. Everything looks fine in 44.0.2. Filed this with Mozilla. See ​https://bugzilla.mozilla.org/show_bug.cgi?id=1249495#c0

Thanks!