Howde, I’m not a coding expert but I managed to resolve this issue for my theme today. Before you change any code, make a back-up of your theme file.
Download a copy of your theme file and save to a folder on your computer. It downloads as a zipped archive so you’ll need to extract that to a folder so that you can search the files. Open the extracted folder and search for overflow: visible - the search will return all the files where overflow: visible; is included; I found 4 files where it was included in my theme code. This helped me to find where exactly the code was present in my whole theme file directory. Going through each of the files where overflow: visible; is present, delete it and then re-run your Pagespeed Insights test to see if the issue is sorted. If it isn’t, then restore the code snippet in the file and move to the next file where it was found.
In my case, the code that was causing the error was in section-main-product.css - deleted it from there and my best practices score went from 77% to 100%. I checked my site on both mobile and desktop and everything looks fine. I can always go back and re-add the code if I later find it has caused some undesirable issue elsewhere.
Hope this helps.