Steps to reproduce:
- go to https://www.mediawiki.org/wiki/Help:Images#Rendering_a_gallery_of_images, scroll down a bit to the 'packed' gallery example
- watch the gallery layout make clean, justified margins
- resize the window horizontally (up or down)
- watch the gallery items reflow, but not resize -- now you have ragged margins
Probably needs some JS code to hook window 'resize' event and adjust packed galleries if horizontal size changes.