New-style packed gallery shows ragged edges after window resize (T55664)

The 'packed' mode for the <gallery> tag, used to display a set of images, is supposed to adjust the image sizes to browser window width, to produce smooth "justified" edges. However, its styling is not corrected after the user resizes their browser. Fix this! :)

The issue affects the mediawiki.page.gallery module.

Please read the complete bug description and comments on the bug report (T55664), as they are often very helpful. Students are required to read Wikimedia's general instructions first.