Tab Button Not Working Properly
I have an iMac later 2015 running Mojave. For some time, hitting tab does not advance to the next field in any browser or app.
iMac Line (2012 and Later)
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Please check if the "Press Tab to highlight each item on a web page" is checked in Safari preferences… > Advanced
Change Advanced preferences in Safari on Mac - Apple Support
Also to further troubleshoot you can check if you have same issue in safe mode.
https://support.apple.com/en-us/HT201262
Note: Safe mode prevents your Mac from loading certain software as it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which are automatically created again as needed.
Hope it helps.