ui: the signal badge is the drag handle (to a plot row, or to reorder); the list rows stop dragging

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/claude/invariants-gtk-and-chrome.md

@@ -245,13 +245,16 @@ Part of [CLAUDE.md](../../CLAUDE.md). These are rules that cost a defect to lear
   alone (`sidebar_wanted`) and the dock's *sensitivity* from `have_signals`; the toolbar's Dock button stays
   sensitive throughout. The wish is persisted (`dock = true|false` in the session file) and a change of it
   schedules a save. `dock:` in the dump reads the result.
-- **An *overlaid* sidebar covers the plot, so a drag out of it has nowhere to land.** `collapsed` in an
-  `AdwOverlaySplitView` means the sidebar sits *on* the plot rather than beside it: the pointer is over the
-  list, never over a drop target, and a signal can be dragged but not dropped. `row_drag_sidebar()` hides
-  the sidebar on `drag-begin` and restores it on `drag-end` — only when collapsed, because side by side the
-  plot is already visible and hiding the list the drag started in would be startling. `pane drag: sidebar
-  hidden for the drag=1 restored after it=1` is the reading; the state is forced collapsed the way the
-  sidebar probe forces it.
+- **The signal badge is the drag handle, and a drop handler only moves widgets.** Dragging a signal onto a plot
+  starts on its badge in the badge bar (a `GtkDragSource` on the badge's button, payload the signal index as a
+  decimal string, taken by the plot area's existing drop target); the list's rows are not drag sources. A drag
+  starting with the slide-over open would have part of the plot hidden under the panel, so the source's
+  `drag-begin` closes it (which only restyles the badges: nothing is destroyed mid-drag). A click without movement
+  never reaches the drag threshold and still opens the page; past it the source claims the sequence and the
+  button's click is cancelled. Each badge is also a `GtkDropTarget` for reordering: `on_badge_drop` calls
+  `gtk_box_reorder_child_after` and rewrites `App::badge_order` from the widgets, and **never rebuilds the badges** --
+  the dragged badge and the target whose `drop` emission is running must both survive it. A foreign drag whose string is not a
+  signal index is refused. `[sigbadges]` reads the controllers, the payload and the reorder; `pane drag:` reads the close.
 - **A stock icon name has to be one *Adwaita* has, and a blank button is the failure mode.** The two
   panes' `[edit]` buttons are `document-edit-symbolic` (the pencil: edit) and `object-select-symbolic`
   (the check: done). The done one was `emblem-ok-symbolic`, which breeze ships and **Adwaita does not** —