Steps to replicate the issue:

• Hover over any MathML formula with existing MathWikiBase reference

What happens?:
Preview pops up at correct height, but at the leftmost place of the website.

What should have happened instead?:
Preview should have pop up right next (over or under) to the formula.

Software version (on Special:Version page; skip for WMF-hosted wikis like Wikipedia):
Math Version 500b9bcefcf399228b67b1dffbd45c50fde9693a

Other information (browser name/version, screenshots, etc.):
Currently, the preview window has the wrong location when hovering over a MathML formula.
The y-location is correct, but the x-location is always at the leftmost position.
This is because the Popup extension needs the values offsetWidth and offsetHeight (the layout size) to work correctly.
[Example image]
Tested in Firefox, Chrome, Edge, Safari.
The location is correct in SVG mode.
There are no warning, etc. in browser console.