correct 5.2: the key was never the cause

docs/workspace-panel-todo.md

@@ -345,9 +345,29 @@ threaten whatever the panel was holding; a panel now holds nothing. A chat panel
 declares free. Fix these for the interaction quality they are genuinely about — scroll position, media
 playback, transcodes — not as a prerequisite for agent coordination._
 
-A panel's React identity is its position plus `key={child.node.id}` on its **nearest ancestor group
-slot** (`WorkspaceRenderer.tsx:178,204`) — the panel's own id is a key nowhere. That's the root cause of
-the whole table:
+**Measured `56ca411`.** `WorkspaceRenderer.test.tsx` mounts the real renderer against a mount-counting
+probe app and lets the real `layout-utils` mutators produce the "after" tree. The table below was written
+from reading the code; the test disagrees with its *diagnosis* in every row, and found one row it had
+missed entirely. Read this paragraph before acting on the bullets underneath it.
+
+- **The key is not the cause.** A panel's React identity is its position plus `key={child.node.id}` on its
+  nearest ancestor group slot, but in each remounting case the **element type** at that position also
+  changes — `PanelSlot` becomes `ResizablePanelGroup`, or the reverse — and React reconciles by type
+  before it looks at keys. Reusing the panel's id so the key does not flip would therefore have changed
+  nothing, at the cost of putting a panel id and a group id into collision, which now matters because a
+  panel id is an agent's address (`agent_panels`).
+- **The row that was missing was the only cheap one, and it is fixed.** `ChildEntry` returned
+  `<>{children}</>` for the first child and `<><Handle/>{children}</>` for every other, so a panel sat in
+  fragment slot 0 when first and slot 1 when not. Remove the leftmost of three panels and the second one
+  found a `ResizableHandle` in its old slot and remounted — for nothing that happened to it. The handle
+  slot is now always present, holding `null` when unneeded.
+- **What is left is genuinely structural.** Wrapping a panel in a group, and a two-child group collapsing
+  to its survivor, both change what kind of thing lives at that position. The only fix that survives
+  contact with React is the one the last bullet of this section already points at: render panels from a
+  host that does not move when the tree reshapes, the way maximize is a CSS toggle on the same element.
+  That is a redesign, not a patch, and it is still Tier C.
+
+The original table, kept because its *observations* hold even where its explanation did not:
 
 | operation                             | remounts?              | why                                                                       |
 | ------------------------------------- | ---------------------- | ------------------------------------------------------------------------- |
@@ -360,10 +380,14 @@ the whole table:
 | mobile panel switch                   | yes, by design         | only the active child renders                                             |
 | **any ephemeral panel on mobile**     | **yes — every panel**  | `WorkspaceView.tsx:165` replaces the workspace instead of overlaying      |
 | **viewport crossing 768px**           | **yes, full**          | `useIsMobile` flips the branch at `WorkspaceView.tsx:165`                 |
+| **a panel stops being second**        | **was yes — fixed**    | `ChildEntry`'s fragment slots shifted under it (`56ca411`)                |
 
-- [ ] **Reuse the panel's id when wrapping it in a new group**, so the ancestor slot key doesn't flip.
-      Kills rows 2 and 3.
-- [ ] **Don't re-key the survivor when a group collapses** (`layout-utils.ts:68-70, 83-88`). Kills row 4.
+- [ ] ~~**Reuse the panel's id when wrapping it in a new group**, so the ancestor slot key doesn't flip.
+      Kills rows 2 and 3.~~ **Withdrawn `56ca411`** — measured, and it kills neither. The element type at
+      that position changes too, which React acts on first. It would also collide a panel id with a group
+      id, and a panel id is an agent's address now.
+- [ ] ~~**Don't re-key the survivor when a group collapses** (`layout-utils.ts:68-70, 83-88`). Kills row
+      4.~~ **Withdrawn `56ca411`**, same reason: the survivor changes type as well as key.
 - [ ] **Overlay the mobile ephemeral panel instead of replacing the workspace**
       (`WorkspaceView.tsx:165`). Affects `/files`, `/email`, `/chat`, `/browser`, `/dashboards`.
 - [ ] **Reference for how it should feel:** maximize (`PanelSlot.tsx:430-457`) is a CSS state toggle on