When you resize the window, if the <gallery> is using the caption option, the caption does not recenter.
Steps to reproduce:
Wikipage with
<gallery caption="Caption here" mode=packed>
Example.png
Example.svg
</gallery>
resize window so gallery needs to be re-flowed.
Originally reported at https://en.wikipedia.org/w/index.php?title=User_talk:Bawolff&oldid=648971666#Gallery_packed_mode_and_caption_centering