mark preserve-sibling-sizes resolved

docs/workspace-panel-todo.md

@@ -479,8 +479,14 @@ work disagree permanently about the roster, with neither told — a direct contr
       the next mutation composes on top of the stale tree. So `refetchOnMount: false` (splitting a panel
       mounts a fresh consumer, which is exactly when a write is in flight), `refetchOnReconnect: false`,
       and `refetchOnWindowFocus` gated on a module-level in-flight count plus a 2 s quiet period.
-- [ ] **Preserve sibling sizes on split.** `splitInner`/`insertPanel` redistribute evenly
+- [x] **Preserve sibling sizes on split.** `splitInner`/`insertPanel` redistribute evenly
       (`100 / newChildren.length`), so one split discards carefully tuned proportions.
+      **Resolved `abea7a3`** — the new sibling takes half of the *target's* size and nothing else moves.
+      One helper serves both call sites, because the drop path (`movePanel` → `insertPanel`) carried the
+      identical bug. Two of the three tests were already in `layout-utils.test.ts` asserting the even
+      split, written to the old behaviour deliberately; they now assert the new one. The third documents
+      the interaction worth knowing: `removePanel` renormalises the group as the panel leaves, so a move
+      reads as renormalise-then-halve, not halve alone.
 - [x] **`tpl-N` panel ids collide across dashboards.** `DashboardPreview.tsx:25-26` —
       `let tplCounter = 0; const tplUid = () => \`tpl-${++tplCounter}\`` — module-level, no entropy,
       resets every page load. Two dashboards created from templates after a reload hold panels with