Fix DocumentMove dialog hiding siblings and nieces/nephews as move targets (outline#11885)

* Initial plan

* fix: show siblings and descendants in DocumentMove dialog

The filterSourceDocument function was incorrectly removing the document's
parent node from the navigation tree, which also hid all siblings (children
of the same parent) and their descendants.

Instead, only the document itself and its own descendants are now excluded
(to prevent circular references). The parent is kept in the tree so siblings
remain visible as valid move targets.

Agent-Logs-Url: https://github.com/outline/outline/sessions/12574f1c-7a7c-45a0-8444-19e24aa10782

Co-authored-by: tommoor <380914+tommoor@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: tommoor <380914+tommoor@users.noreply.github.com>

Author: Copilot

app/components/DocumentExplorer/DocumentMove.tsx

@@ -3,6 +3,7 @@ import { useState, useMemo } from "react";
 import { useTranslation, Trans } from "react-i18next";
 import { toast } from "sonner";
 import type { NavigationNode } from "@shared/types";
+import { descendants, flattenTree } from "@shared/utils/tree";
 import type Document from "~/models/Document";
 import Button from "~/components/Button";
 import Text from "~/components/Text";
@@ -23,13 +24,23 @@ function DocumentMove({ document }: Props) {
   const [selectedPath, selectPath] = useState<NavigationNode | null>(null);
 
   const items = useMemo(() => {
-    // Recursively filter out the document itself and its existing parent doc, if any.
+    // Collect the IDs of the document itself and all of its descendants so they
+    // can be excluded from the move targets (moving to self or a descendant
+    // would create a cycle; moving to the exact same location is a no-op).
+    const allNodes = collectionTrees.flatMap(flattenTree);
+    const sourceNode = allNodes.find((node) => node.id === document.id);
+    const excludedIds = new Set<string>([document.id]);
+    if (sourceNode) {
+      descendants(sourceNode).forEach((n) => excludedIds.add(n.id));
+    }
+
+    // Recursively filter out the document itself and its descendants.
+    // The document's current parent is intentionally kept so that siblings
+    // remain visible as valid move targets.
     const filterSourceDocument = (node: NavigationNode): NavigationNode => ({
       ...node,
       children: node.children
-        ?.filter(
-          (c) => c.id !== document.id && c.id !== document.parentDocumentId
-        )
+        ?.filter((c) => !excludedIds.has(c.id))
         .map(filterSourceDocument),
     });
 
@@ -43,7 +54,7 @@ function DocumentMove({ document }: Props) {
       );
 
     return nodes;
-  }, [policies, collectionTrees, document.id, document.parentDocumentId]);
+  }, [policies, collectionTrees, document.id]);
 
   const move = async () => {
     if (!selectedPath) {