Thanks:
RH-77893 Show Toolbar slow / glitchy
I have no idea. Somewhere along the way, RUI files kinda became Libraries. I don’t think it’s consistent throughout the UI, though.
RH-77894 “Libraries” vs “RUI Files”
You’re not actually simply moving in this case. You’re dragging the tab out of the container, dropping it on empty space. The old container is destroyed and a new one is created. It’s easier to see what’s happening if you do the same maneuver on a container with multiple toolbar tabs. If you want it to just move, drag the “…” gripper (the darker gray part to the right of the tab).