Many themes may not have styles in them that expect galleries to appear in a widget context. As noted in §Default Themes Updates for Media Widgets for Images, Video, and Audio:
Themes that add custom styles to the MediaElement.js player (namely Twenty Thirteen and Twenty Fourteen) were updated from just styling it within syndicated content, to also include instances within widgets. Most themes don’t restrict styles for captioned images or media players to just post content, that is, limit CSS selectors to classes output by post_class(). If your theme does, make sure to either remove that constraint or include a .widget selector.
We need to make sure that all of the core themes are compatible, and also come up with common changes that themes may need to make to ensure they're compatible. This can then be put into a dev note.
westonruter
commented
7 years ago
Many themes may not have styles in them that expect galleries to appear in a widget context. As noted in §Default Themes Updates for Media Widgets for Images, Video, and Audio:
We need to make sure that all of the core themes are compatible, and also come up with common changes that themes may need to make to ensure they're compatible. This can then be put into a dev note.