I took two screenshots, shown below. If you open them in your browser, you can see the setting pop up on two different X axis. I think this is why it's shaking as OP stated.

Thanks for saving us with this extension 🙏 I have experienced the setting shown twice in different left pos on different browsers (Chrome, Edge, Brave), on two devices with latest Tabox. The monitors are all 1920x1080 FHD displays. It's basically showing the setting twice or more, horizontally (different left pos) - as if you are seeing double. On one, I was able to fix it by changing the Zoom factor in the browser's setting. The others are default (medium, 100%). I have also disabled all extensions but Tabox. The setting box still shows up more than twice which makes it unusable. I've looked at your "https://www.tabox.co/using-tabox" Using Tabox but I don't see the setting menu on it, otherwise, I can at least "try" to guess which is which. Finally, Yes, I tried it on Incognito mode and having the same result. The funny thing is that if I took a screen shot, it came out as only one. It's as if in a split second, the setting menu is redrawing multiple time at different left pos. Hope this gives more info so you can get it resolved. Thanks!!