Add tab re-ordering.

README.md

@@ -109,7 +109,7 @@ just the VT core.
 ```
 main.zig       AdwApplication startup
 appearance.zig the colour scheme: preference -> libadwaita, CSS, palette
-Window.zig     sidebar + GtkStack of views, tab management, shortcuts
+Window.zig     sidebar + GtkStack of views, tab management, reordering, shortcuts
 View.zig       one tab's content: its panes, their layout, and drag handling
 Layout.zig     the split tree: nodes, rearranging, GtkPaned materialization
 Pane.zig       content plus its header, drag source, and drop target
@@ -168,6 +168,19 @@ preview. Cancelling a drag puts the pane back: only the dragged pane ever
 moves, so the rest of the tree is unchanged and re-inserting it beside its
 original sibling restores the original shape.
 
+**Tabs reorder the same way.** Dragging a sidebar row moves the tab, and the
+rows shuffle under the pointer as you go rather than waiting for the drop; a
+copy of the row travels with the cursor while the one being moved stays dimmed
+where it currently sits. Dropping in the space below the last row puts the tab
+at the end, and a drag that ends anywhere else — over a pane, off the window —
+is a cancel, which puts the tab back at the index it started from.
+
+The order is the tab list itself, which is the order everything else already
+reads: `Ctrl+1`..`Ctrl+9`, next/previous tab, and the startup list that "use
+current tabs" captures. The sidebar follows it through a sort function rather
+than by moving rows around, since taking a row out of a `GtkListBox` to put it
+back elsewhere would drop the selection and the focus with it.
+
 ## Layouts
 
 A **layout** is a saved tab: an arrangement of panes, each with a directory and
@@ -364,7 +377,8 @@ in principle, but a terminal grid is small.
 - Resize reflows the grid and notifies the child
 - Window title (OSC 0/2) becomes the pane header and tab label; the tab shows
   its pane count once a view holds more than one
-- Tabs: create, close, switch; closing the last one closes the window
+- Tabs: create, close, switch, and reorder by dragging a sidebar row, with the
+  rows shuffling live as you drag; closing the last one closes the window
 - Multiple panes per tab in an arbitrary split tree, rearranged by keyboard
   or by dragging a pane's header, with draggable dividers between them;
   closing the last pane in a view closes its tab