I created a feature where a floating button appears at the end of the selection when text is selected.
When the selection reached the end of the screen, the floating button automatically switched placement to the right, but it seemed to have a lower layer than the system UI representing the selection and was not clickable.
https://i.stack.imgur.com/FtmZN.jpg
This was a problem in iOS Safari, and it was hard to find examples of how to fix it. I didn't check Android as I don't have a device.
No have this issue in Chrome, Safari, or Firefox on the desktop environment.