This is with Chrome

Just wanted to clarify. I get no errors when I'm on the home page, but I do get errors in the console when on the enwiki main page.

Cannot reproduce in Chromium 80, results are shown.
Does this also happen when disabling any browser add-ons and using a new browser profile?
Does this also happen in another browser (which one)?
Which entries does the "Network" tab of Chrome's developer tools show when entering a search query into the search field?
See https://developers.google.com/web/tools/chrome-devtools for Chrome and Chromium

I disabled the only non-Google browser add-on that I have and it didn't make a difference. I tried using Safari and have no problem. I tried using the Chrome guest profile, and the problem is also resolved. I'm not sure why this is, since I hadn't changed any of my Chrome profile settings before this issue started.

If I'm understanding you correctly, when I enter something in the search box inside the developer window when on the Wikipedia homepage, I get the error: error: {code: "badinteger", info: "Invalid value "176.00000381469727" for integer parameter "pithumbsize".",…}

I do not know what you mean by complete error. The URL is https://www.wikipedia.org. I have tried entering many search terms. For example, one that does not work is entering "Wikipedia" in the search field. I do not press enter after typing it in, so I don't know what other URL you are referring to.

Edit: Forgive me, do you mean the link from the developer console? If so, it is: https://en.wikipedia.org/w/api.php?action=query&format=json&generator=prefixsearch&prop=pageprops%7Cpageimages%7Cdescription&redirects=&ppprop=displaytitle&piprop=thumbnail&pithumbsize=176.00000381469727&pilimit=6&gpssearch=Wikip&gpsnamespace=0&gpslimit=6&callback=callbackStack.queue%5B5%5D.
There are many errors listed, but it looks like they're all the same. The full error that prints out at that link is:
/**/callbackStack.queue[5]({"error":{"code":"badinteger","info":"Invalid value \"176.00000381469727\" for integer parameter \"pithumbsize\".","*":"See https://en.wikipedia.org/w/api.php for API usage. Subscribe to the mediawiki-api-announce mailing list at <https://lists.wikimedia.org/mailman/listinfo/mediawiki-api-announce> for notice of API deprecations and breaking changes."},"servedby":"mw1396"})

Thanks, that comment is welcome!

@Ergo_Sum thanks. That error message actually does help a lot.

What's happening there is that the API is requesting a thumbnail with the size 176.00000381469727px, which is invalid because the API expects that number to be an integer. The entire API response fails at that points and doesn't return any results.

The code determining that number is located on mw-typeahead.js#29:

thumbnailSize = getDevicePixelRatio() * 80

Which requests a thumbnail that is relative to pixel density of the users device. So if the pixel density is not an integer (in this case the pixel density is 2.2) then the search typeahead fails! The solution is to wrap that value in parseInt().

I'm surprise this bug hasn't been raised earlier...

This sounds like a problem that is not possible for me to fix, no?

@Jdrewniak Thank you.

Change 585489 had a related patch set uploaded (by Jdrewniak; owner: Jdrewniak):
[wikimedia/portals@master] Ensure typeahead thumbnail size is an integer

https://gerrit.wikimedia.org/r/585489

Change 585489 merged by jenkins-bot:
[wikimedia/portals@master] Ensure typeahead thumbnail size is an integer

https://gerrit.wikimedia.org/r/585489

@Ergo_Sum this should be resolved now, please confirm and we can close out the ticket.

@Jdrewniak Yes, it is resolved now. Thank you.