@Jdlrobson I tried the code you provided in my local common.js, and I also tried executing it on window.load (since the JQuery load event handler can be flaky) but was still able to reproduce the issue.

This might not be a great workaround, but if I'm understanding the purpose of the positionBanner() method correctly, the method can be modified to exit immediately if both of the following are zero, correct?

centerX = $wpbBannerImage.data( 'pos-x' ),
centerY = $wpbBannerImage.data( 'pos-y' );

The bug will still be there for banners that use these parameters, but since those are a tiny subset of all banners it should at least resolve the problem until a better fix is found.

Change 236735 had a related patch set uploaded (by Sumit):
WikidataPageBanner set focus after banner load

https://gerrit.wikimedia.org/r/236735

@Jdlrobson, I've added a fix, but since I'm unable to see the issue locally, I'll need your support for testing.
@Wrh2, I've tried testing the fix on Wikivoyage, by adding the code at https://en.wikivoyage.org/wiki/User:Sumit.iitp/common.js It seems to work and I've added the above patch along those lines. You can copy the same code to your User's common.js and verify if my assumption is true. It will help in attesting the correctness of the patch :)

@Sumit - see my second comment on this ticket. I've tried updating my common.js file with the Javascript Jdlrobson suggested, as well as the Javascript you provided, but in both cases I'm still able to reproduce the problem. It appears that either Firefox is firing the onload event before the image is loaded or else JQuery is failing at a height calculation early in the load process.

If I disable Javascript then everything seems to work fine, so this seems to be a problem specific to the positionBanner() method. Am I correct in the belief that the CSS this method appends is only needed when the "pos-x" and "pos-y" values are non-zero? If so I think the method can exit without changing any CSS if those values are zero, which should be a good temporary fix for the majority of pages while the underlying issue is addressed.

Change 236735 merged by jenkins-bot:
WikidataPageBanner set focus after banner load

https://gerrit.wikimedia.org/r/236735

@Wrh2, the above fix has been deployed on English Wikivoyage. To test, I tried repeated refreshing after cache clearing each time on https://en.wikivoyage.org/wiki/Bertha_Benz_Memorial_Route. However I wasn't able to reproduce the issue. If the issue still persists, let me know the pages on which I can reproduce and see. Otherwise, I'll close this task :)

I was unable to reproduce the issue just now, but have also asked for feedback from the original reporters at https://en.wikivoyage.org/wiki/Wikivoyage:Travellers'_pub#banner_not_displaying.

Thanks for the quick attention.

The individual who originally reported this issue now also says it is fixed. Thanks again Sumit - this task can now be closed.