[idea] Show a tab moving while you drag it to reorder

[abdh01h]

Reordering tabs exists, but silently: press a tab, release it on another, and it moves. Between the press and the release nothing changes on screen, so it reads as nothing happening until the tab jumps — I only found it by accident.

The Spaces sidebar already shows this well: the space being dragged dims and a line marks the edge where it will land.

I'd like the tab bar to do the same: hold a tab and move the pointer, see that tab dim, with an insertion line between the tabs where it would land, then drop it there.

Moving a tab onto a space in the sidebar is a separate idea (#5312), and the wider drag-and-drop picture is #1534. This one is only about seeing a tab reorder in the tab bar while dragging it.