I'm guessing that there is either a custom stylesheet loaded, or maybe a browser extension conflicting whenever you are logged in. It's hard to tell unless I can see it.
There are maybe two things you can try:
1. Disable animations: https://i.imgur.com/VokVnfJ.png
If there is something affecting the animations, then this will very likely help.
2. Disable the scroll bar script: https://i.imgur.com/A1WrTqT.png
Let's hope this does change something.Best,
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