Customize themes section doesn't show theme screenshots, border hover issues

[celloexpressions]

There are some odd visual regressions in the themes section in trunk. This'll be replaced in #37661, but this ticket is to track those in case that project doesn't make it into 4.7. See ​https://wordpress.slack.com/archives/core-customize/p1475517778001564.

[celloexpressions]

No idea what happened here, but invalidated by [38813].

[ocean90]

Needs a patch because of [39140].

[mckernanin]

I did some digging into the missing screenshots.
It appears that renderScreenshots() is no longer being called on opening the panel. In 4.6.1, this was done in onChangeExpanded(), line 977. renderScreenshots() is still bound to the filter field, if you type something into the theme search box then all of the screenshots will load.
I'm not sure where this should be hooked back in, but I'm happy to assist however I can!

Related to [34391]

[delawski]

@celloexpressions - Just as @mckernanin noticed, Themes panel's expand routine is missing a call to renderScreenshots() and scroll event listener. I've added missing lines and issued a PR:

​https://github.com/xwp/wordpress-develop/pull/195

There's also a diff file attached to this ticket.

[delawski]

I've completely missed the styling issues in my initial patch. I've just submitted a new patch that fixes both screenshots rendering and CSS issues (​the PR is updated as well)

[mckernanin]

Patch looks good, fixes both issues.

[westonruter]

In 39153:

Customize: Fix loading of theme screenshots in themes section and remove erroneous borders on hover.

Fixes regressions introduced in [38648].

Props delawski, mckernanin.
See #34391.
Fixes #38222.