fix(ui): spell-bar drag-reorder works — the per-frame rebuild was destroying the dragged cell (#354)

Everything already existed — the drag payloads, the favorite wire pair
(0x1E3 add-at-position / 0x1E4 remove, byte-confirmed against retail's
Event_AddSpellFavorite @0x006A0F70 and ACE), the insert-shift state
ops. The bug: lifting a favorite fires SpellbookChanged, the next
per-frame Tick rebuilt the bar, the rebuild flushed and recreated
every cell, and UiRoot's subtree-removal safety net canceled the
in-flight drag whose source had just been destroyed — one frame after
every lift, before any drop could land.

The rebuild now defers for the duration of the drag gesture, and the
drop ports retail's own -1-if-lifted-before-target index adjustment
(SpellCastSubMenu::AddFavorite @0x004C7060) so final positions are
byte-identical: insert-shift, not swap; drag-out still deletes (the
lift's removal stands on a missed drop, retail's shape). The
real-pointer-pipeline test fails against the pre-fix code with the
exact cancellation and passes after; a discriminator pins that
physical-item drop handlers reject the spell payload.

AP-172 files the one presentation divergence (mid-drag reflow happens
on release, not continuously) — renumbered from the agent's AP-171
draft, which collided with the same-day double-click row. #354 filed
and closed.

Clean-room complete solution: 11,541 passed / 4 skipped / 0 failed.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/AcDream.App/UI/Layout/SpellcastingUiController.cs

@@ -66,6 +66,7 @@ public sealed class SpellcastingUiController : IRetainedPanelController
     private bool _disposed;
     private bool _favoritesDirty;
     private bool _endowmentDirty;
+    private bool _favoriteDragActive;
 
     private SpellcastingUiController(
         ImportedLayout layout,
@@ -422,16 +423,43 @@ public sealed class SpellcastingUiController : IRetainedPanelController
     }
 
     private void BeginFavoriteDrag(SpellFavoriteDragPayload payload)
-        => _removeFavorite?.Invoke(payload.SourceTab, payload.SpellId);
+    {
+        // gmSpellcastingUI::RecvNotice_ItemListBeginDrag @ 0x004C7360 removes the
+        // lifted item from PlayerModule (+ sends the wire RemoveSpellFavorite)
+        // the instant the drag starts. That removal fires SpellbookChanged, which
+        // would normally set _favoritesDirty and let the next Tick() rebuild the
+        // whole favorite list -- but Rebuild() flushes and recreates every slot
+        // (UiItemList.Flush -> RemoveChild), and UiRoot's subtree-removal safety
+        // net (ClearSubtreeOwnership) cancels any drag rooted in a removed
+        // element. Left alone, a per-frame Tick() would destroy the very cell
+        // driving this gesture and silently cancel the reorder before the user
+        // can complete the drop. _favoriteDragActive defers the rebuild for the
+        // gesture's duration; DropFavorite compensates target indices for the
+        // now-stale numbering (retail's own -1-if-lifted-before-target rule,
+        // ported below).
+        _favoriteDragActive = true;
+        _removeFavorite?.Invoke(payload.SourceTab, payload.SpellId);
+    }
 
-    private static void EndFavoriteDrag(SpellFavoriteDragPayload payload)
+    private void EndFavoriteDrag(SpellFavoriteDragPayload payload)
     {
         // The press-time command already performed retail's PlayerModule
-        // removal on the one Runtime-owned Spellbook.
+        // removal on the one Runtime-owned Spellbook. Release the rebuild
+        // deferral -- the next Tick() resyncs the list to the (possibly
+        // further-updated-by-Drop) Spellbook state.
+        _favoriteDragActive = false;
     }
 
     private void DropFavorite(SpellFavoriteDragPayload payload, int targetTab, int targetPosition)
     {
+        // Rebuild() was deferred for the whole gesture (see BeginFavoriteDrag), so
+        // every sibling slot's captured target index is still numbered against the
+        // PRE-lift list. Retail's own SpellCastSubMenu::AddFavorite @ 0x004C7060
+        // corrects for exactly this staleness: when the lifted item's original
+        // index was before the drop target, the target index shifts down by one
+        // to land where the target visually sits once the gap closes.
+        if (payload.SourceTab == targetTab && payload.SourcePosition < targetPosition)
+            targetPosition -= 1;
         _addFavorite?.Invoke(targetTab, targetPosition, payload.SpellId);
         _selected[targetTab] = payload.SpellId;
     }
@@ -470,7 +498,7 @@ public sealed class SpellcastingUiController : IRetainedPanelController
             UpdateEndowment();
             SelectTab(_activeTab);
         }
-        if (_favoritesDirty)
+        if (_favoritesDirty && !_favoriteDragActive)
         {
             _favoritesDirty = false;
             Rebuild();