The <gallery> tag generation has been updated to include srcset attributes
for high-density displays: <https://phabricator.wikimedia.org/T64709>
An unfortunate consequence is that if extensions have parser test cases
including a <gallery> they will need to be updated for the new HTML.
The only one I noticed on my setup with in Cite, so I submitted a patch to
fix that; but keep an eye out for surprise test failures elsewhere.
It's an easy fix -- run the parser tests, find the failing test, and
copy-paste the 'srcset' attribute from the generated HTML into the test
case expected output.
-- brion