I did test it (on the screen) in Windows 10, with SSRS 2016 and Edge version 89.0.774.54 (Official build) (64-bit) as well as chrome Version 89.0.4389.90 (Official Build) (64-bit).
Now I am wondering if it is the SSRS version as you are on 2012 and I am on 2016 (didn't notice this previously).
I wouldn't expect that to make a difference as it appears to be a rendering issue (which is a browser issue, not a server side issue), but maybe they have some back-end style changes between 2012 and 2016 that cause weird issues with the dropdowns?
As much as "upgrade" isn't an ideal recommendation, 2012 is out of support (as is 2016), so upgrading to a supported version may not be a bad idea anyway.
The above is all just my opinion on what you should do.
As with all advice you find on a random internet forum - you shouldn't blindly follow it. Always test on a test server to see if there is negative side effects before making changes to live!
I recommend you NEVER run "random code" you found online on any system you care about UNLESS you understand and can verify the code OR you don't care if the code trashes your system.