fix(17): WR-02 keep one reorder save in flight and ignore stale responses

A move made while a save runs is queued and sent after it settles, so
out-of-order completion can no longer leave the database on an older tree
than the screen. A failure that a waiting save supersedes keeps the later
moves, and a response tied to an earlier load or partition leaves the
current tree and its last saved state alone.

admin/src/views/ReorderView.vue

@@ -30,9 +30,13 @@ import { showToast } from '../state/useToasts'
 // or inside a row, never into the node's own subtree), by the arrow keys on
 // the handle and by the four buttons. There is no Save button: every move is
 // saved by one POST with the whole scoped node set, 400 ms after the last
-// move, and a pending save is flushed when the page is left. A failed save
-// restores the last saved tree; a 409 (the records changed meanwhile)
-// reloads it.
+// move, and a pending save is flushed when the page is left. Only one save is
+// in flight at a time: a save asked for meanwhile waits for it and then sends
+// the tree as it was when asked, so the server never ends on an older tree
+// than the screen. A failed save restores the last saved tree unless a later
+// save (which carries the whole tree) is waiting; a 409 (the records changed
+// meanwhile) reloads it. A response that arrives after the page reloaded or
+// switched partition leaves the screen alone.
 const SAVE_DEBOUNCE_MS = 400
 const SAVED_SHOWN_MS = 2000
 
@@ -59,7 +63,17 @@ const saving = ref(0)
 const savedShown = ref(false)
 const announcement = ref('')
 
+// One save: the tree it sends, its partition and the load generation it
+// was taken from.
+interface SaveJob {
+  sent: TreeNode[]
+  partition: AdminReorderPartition['value'] | null
+  generation: number
+}
+
 let saveTimer: ReturnType<typeof setTimeout> | null = null
+let inFlight = false
+let queued: SaveJob | null = null
 let savedTimer: ReturnType<typeof setTimeout> | null = null
 let announceTimer: ReturnType<typeof setTimeout> | null = null
 let generation = 0
@@ -106,31 +120,72 @@ function showSaved(): void {
   }, SAVED_SHOWN_MS)
 }
 
-async function save(): Promise<void> {
+/** Saves the tree as it is now, or queues it behind the save in flight. */
+function save(): void {
   saveTimer = null
-  const sent = cloneTree(tree.value)
-  const body = { nodes: flatten(sent), partition: partition.value?.value ?? null } as unknown as AdminReorderRequest
+  const job: SaveJob = { sent: cloneTree(tree.value), partition: partition.value?.value ?? null, generation }
+  if (inFlight) {
+    queued = job
+    return
+  }
+  void send(job)
+}
+
+/** Whether a later move of the same page is waiting to be saved. */
+function superseded(job: SaveJob): boolean {
+  return saveTimer !== null || (queued !== null && queued.generation === job.generation)
+}
+
+function failed(job: SaveJob): void {
+  if (job.generation === generation) {
+    if (superseded(job)) {
+      // The waiting save carries the whole tree, later moves included.
+      return
+    }
+    tree.value = cloneTree(savedTree)
+  }
+  showToast(t('backend::lang.reorder.failed'), 'danger')
+}
+
+async function send(job: SaveJob): Promise<void> {
+  inFlight = true
+  const body = { nodes: flatten(job.sent), partition: job.partition } as unknown as AdminReorderRequest
   savedShown.value = false
   saving.value++
   try {
     const { data, response } = await api.POST('/{vendor}/{plugin}/{controller}/reorder', { params: { path }, body })
+    const current = job.generation === generation
     if (data) {
-      savedTree = sent
-      showSaved()
+      if (current) {
+        savedTree = job.sent
+        if (!superseded(job)) {
+          showSaved()
+        }
+      }
       return
     }
     if (response.status === 409) {
-      showToast(t('backend::lang.reorder.stale'), 'danger')
-      await load()
+      if (current) {
+        // A waiting save of the same set would be refused too.
+        if (queued !== null && queued.generation === job.generation) {
+          queued = null
+        }
+        showToast(t('backend::lang.reorder.stale'), 'danger')
+        await load()
+      }
       return
     }
-    tree.value = cloneTree(savedTree)
-    showToast(t('backend::lang.reorder.failed'), 'danger')
+    failed(job)
   } catch {
-    tree.value = cloneTree(savedTree)
-    showToast(t('backend::lang.reorder.failed'), 'danger')
+    failed(job)
   } finally {
     saving.value--
+    inFlight = false
+    const next = queued
+    queued = null
+    if (next) {
+      void send(next)
+    }
   }
 }
 
@@ -138,14 +193,14 @@ function scheduleSave(): void {
   if (saveTimer !== null) {
     clearTimeout(saveTimer)
   }
-  saveTimer = setTimeout(() => void save(), SAVE_DEBOUNCE_MS)
+  saveTimer = setTimeout(save, SAVE_DEBOUNCE_MS)
 }
 
 /** Sends a pending save at once (leaving the page, changing the partition). */
 function flush(): void {
   if (saveTimer !== null) {
     clearTimeout(saveTimer)
-    void save()
+    save()
   }
 }