If your goal is to have two-dimensional, always visible scrollbars, currently Flet doesn’t support this feature out of the box. I ran into the same limitation, so I created an extension for Flet that adds true two-dimensional scrollbars and some additional functionality. It might help in your case: Flet-Extended-Interactive-Viewer.

With normal Flet, you can have two scrollbars, but one of them only appears at the end of the other, which isn’t ideal for some layouts.