Thanks for looking into this, works just fine!

When walking past the top or bottom of the list the active item is clamped to the first or last item. For example, pressing up on the third item on the first row, the first item will be activated. The alternate behavior could be to not take action on up or down arrow when on the first or last row.

That's how the File Browser works too, and I think that's fine.
Plus, in this case trying to wrap around would unveil a tricky issue: To change the pose when a new item is activated (uiListDyn.custom_activate_optype), we need the button, or better said the button's constext-store (which stores the asset-handle it represents). However the UI-list code only creates the buttons that are actually visible, so for the handling on a wrap around to work, we'd have to force an immediate redraw or find some other solution, so the button is created in time.

This allows using left and right to navigate up and down regular UI lists. It would be simple to prevent this, but I don't see any issue with this.

Don't see an issue with that either. Again, the File Browser works like this too.