Thanks for reaching out about this issue you're seeing in Safari. I'll be happy to help out.
If you haven't restarted your Mac yet, try that now to see if it clears the issue. If it doesn't, test out to see if this happens in safe mode. Here's some info about starting in safe mode and how it can help with these issues: Use safe mode to isolate issues with your Mac - Apple Support
If you're still having the same trouble, let us know your findings starting safe mode.
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Weird banding opening new tabs in Safari Mojave
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