fix(tree): stop silent page loss on move-to-unloaded-parent + reconnect ghost roots (#159)

Two confirmed P1 data-loss findings in the sidebar tree sync.

#1 — Move into an unloaded/collapsed parent silently dropped pages. When a
moveTreeNode (or addTreeNode) broadcast targeted a parent whose children were
NOT yet lazy-loaded, `insertByPosition` did `kids = parent.children ?? []` and
inserted the moved node, MATERIALIZING a misleading partial child list
(`[movedNode]`) out of an unloaded (`children === undefined`) parent. The
lazy-load gate fetches only when children are absent/empty, so it then refused
to fetch — leaving the parent showing ONLY the moved node and HIDING all its
other real children (and, when the parent wasn't in the tree at all, the node
was removed and never re-fetched). Fix: `insertByPosition` distinguishes
`children === undefined` (not loaded) from `[]` (loaded-empty) and, for an
unloaded parent, does NOT insert — it leaves children unloaded and just flags
`hasChildren`, so expanding fetches the FULL set (including the moved/added
node) via the existing lazy-load.

#2 — After a socket reconnect, a deleted/moved-away root lingered as a 404
"ghost". `mergeRootTrees` was append-only: it kept every previously-loaded root
and only added new ones, so a root removed during the missed-events gap was
never dropped. It runs only once all root pages are fetched, so the incoming
list is the authoritative complete root set — fix reconciles to it (drop roots
absent from incoming) while PRESERVING each surviving root's lazy-loaded
subtree and refreshing its own fields.

Tests: insertByPosition unloaded-vs-loaded-empty parent; the move reducer
keeps a collapsed destination lazy-loadable instead of partial; mergeRootTrees
drops a ghost root, preserves a surviving subtree, adds new roots, refreshes
fields. The existing "remove when parent not in tree" reducer test still holds.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/client/src/features/page/tree/utils/utils.test.ts

@@ -6,6 +6,7 @@ import {
   collectBranchIds,
   openBranches,
   closeIds,
+  mergeRootTrees,
 } from "./utils";
 import type { IPage } from "@/features/page/types/page.types.ts";
 import type { SpaceTreeNode } from "@/features/page/tree/types.ts";
@@ -44,10 +45,7 @@ function flatNode(
 }
 
 // Nested SpaceTreeNode factory for collectAllIds / collectBranchIds.
-function treeNode(
-  id: string,
-  children: SpaceTreeNode[] = [],
-): SpaceTreeNode {
+function treeNode(id: string, children: SpaceTreeNode[] = []): SpaceTreeNode {
   return {
     id,
     slugId: `slug-${id}`,
@@ -94,11 +92,7 @@ describe("collectBranchIds", () => {
       ]),
       treeNode("root2", [treeNode("leaf3")]),
     ];
-    expect(collectBranchIds(tree).sort()).toEqual([
-      "branch1",
-      "root",
-      "root2",
-    ]);
+    expect(collectBranchIds(tree).sort()).toEqual(["branch1", "root", "root2"]);
   });
 
   it("returns [] for a leaf-only tree", () => {
@@ -273,3 +267,57 @@ describe("closeIds", () => {
     expect(twice).toEqual({ keep: true, a: false, b: false });
   });
 });
+
+describe("mergeRootTrees (#159 #2 reconnect reconcile)", () => {
+  // Root node with a position and optional already-loaded children.
+  function root(
+    id: string,
+    position: string,
+    children?: SpaceTreeNode[],
+  ): SpaceTreeNode {
+    return {
+      id,
+      slugId: `slug-${id}`,
+      name: id.toUpperCase(),
+      icon: undefined,
+      position,
+      spaceId: "space-1",
+      parentPageId: null as unknown as string,
+      hasChildren: !!children?.length,
+      children: children as SpaceTreeNode[],
+    };
+  }
+
+  it("DROPS a stale root that is absent from the incoming (authoritative) set", () => {
+    // 'ghost' was a root before the gap; the server's current roots no longer
+    // include it (deleted / moved under another page). It must not linger.
+    const prev = [root("a", "a0"), root("ghost", "a2"), root("b", "a4")];
+    const incoming = [root("a", "a0"), root("b", "a4")];
+    const merged = mergeRootTrees(prev, incoming);
+    expect(merged.map((n) => n.id)).toEqual(["a", "b"]);
+    expect(merged.find((n) => n.id === "ghost")).toBeUndefined();
+  });
+
+  it("PRESERVES a surviving root's lazy-loaded children (subtree not lost on refetch)", () => {
+    const loadedChild = root("a1", "a0");
+    const prev = [root("a", "a0", [loadedChild])];
+    // The root query returns only top-level roots (no children).
+    const incoming = [root("a", "a0")];
+    const merged = mergeRootTrees(prev, incoming);
+    expect(merged[0].children?.map((c) => c.id)).toEqual(["a1"]);
+  });
+
+  it("ADDS a new incoming root", () => {
+    const prev = [root("a", "a0")];
+    const incoming = [root("a", "a0"), root("new", "a2")];
+    const merged = mergeRootTrees(prev, incoming);
+    expect(merged.map((n) => n.id)).toEqual(["a", "new"]);
+  });
+
+  it("REFRESHES a surviving root's own fields from the incoming copy (e.g. rename)", () => {
+    const prev = [{ ...root("a", "a0"), name: "OLD" }];
+    const incoming = [{ ...root("a", "a0"), name: "NEW" }];
+    const merged = mergeRootTrees(prev, incoming);
+    expect(merged[0].name).toBe("NEW");
+  });
+});