feat(ui): the spell-bar drop ring — retail's authored drag-accept state, and the ring exposed a real drop off-by-one

The green ring is retail's own art: every UIItem cell carries an
authored DragAccept child (catalog 0x21000037, child 0x1000045A), and
the spell bar's drag-over handler (SpellCastSubMenu::OnItemListDragOver
@0x004C5990) flips it to the Accept state (0x10000040 -> surface
0x060011F9) for any spell payload. Ported through a per-slot
SetDragAcceptVisual seam + a catalog DragOverAcceptance hook; other
lists are untouched (null acceptance = neutral). A polarity error in
our older docs (Accept/Reject state ids swapped) was corrected against
three independent sources; the shipped art was always right, only the
labels lied.

The ring shares ONE landing computation with the drop
(FavoriteDropIndex) — and that requirement exposed a genuine #354
off-by-one: the empty-tail path double-applied the -1 adjustment
(retail gates it on the lift's removal @0x004C7157), landing a
reordered spell second-to-last instead of last. Fixed;
discriminator-verified both ways. AP-172 narrowed + its false
empty-tail claim corrected.

Clean-room complete solution: 11,545 passed / 4 skipped / 0 failed.

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

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

@@ -349,7 +349,8 @@ public sealed class SpellcastingUiController : IRetainedPanelController
                     uint id = spellId;
                     int position = list.GetNumUIItems();
                     _spellbook.TryGetMetadata(id, out SpellMetadata? metadata);
-                    var slot = new UiCatalogSlot
+                    UiCatalogSlot? slot = null;
+                    slot = new UiCatalogSlot
                     {
                         EntryId = id,
                         CatalogIconTexture = metadata is null ? 0u : _resolveSpellIcon(id),
@@ -358,10 +359,11 @@ public sealed class SpellcastingUiController : IRetainedPanelController
                         CatalogDragPayload = new SpellFavoriteDragPayload(tab, position, id),
                         DragBegan = payload => BeginFavoriteDrag((SpellFavoriteDragPayload)payload),
                         DragEnded = payload => EndFavoriteDrag((SpellFavoriteDragPayload)payload),
+                        DragOverAcceptance = FavoriteDragOverAcceptance,
                         Dropped = payload =>
                         {
                             if (payload is SpellFavoriteDragPayload favorite)
-                                DropFavorite(favorite, targetTab, position);
+                                DropFavorite(favorite, targetTab, list, slot!);
                             else if (payload is SpellbookShortcutDragPayload shortcut)
                                 DropSpellbookShortcut(shortcut, targetTab, position);
                         },
@@ -386,15 +388,17 @@ public sealed class SpellcastingUiController : IRetainedPanelController
         slot = new UiCatalogSlot
         {
             SpriteResolve = list.SpriteResolve,
+            DragOverAcceptance = FavoriteDragOverAcceptance,
             Dropped = payload =>
             {
-                int position = Math.Max(0, list.IndexOf(slot!));
-                int favoriteCount = _spellbook.GetFavorites(targetTab).Count;
-                position = Math.Min(position, favoriteCount);
                 if (payload is SpellFavoriteDragPayload favorite)
-                    DropFavorite(favorite, targetTab, position);
+                    DropFavorite(favorite, targetTab, list, slot!);
                 else if (payload is SpellbookShortcutDragPayload shortcut)
+                {
+                    int position = Math.Max(0, list.IndexOf(slot!));
+                    position = Math.Min(position, _spellbook.GetFavorites(targetTab).Count);
                     DropSpellbookShortcut(shortcut, targetTab, position);
+                }
             },
         };
         ConfigureShortcutOverlay(slot, shortcutIndex);
@@ -450,16 +454,54 @@ public sealed class SpellcastingUiController : IRetainedPanelController
         _favoriteDragActive = false;
     }
 
-    private void DropFavorite(SpellFavoriteDragPayload payload, int targetTab, int targetPosition)
+    /// <summary>
+    /// SpellCastSubMenu::OnItemListDragOver @ 0x004C5990: while a drag hovers a
+    /// favorite-bar cell (occupied OR empty — retail's tail cells are UIItems in
+    /// the same list), retail sets the authored per-cell DragAccept child
+    /// (element 0x1000045A, bound in UIElement_UIItem::PostInit @ 0x004E1870)
+    /// to ItemSlot_DragOver_Accept (UIStateId 0x10000040 → authored ring
+    /// 0x060011F9) when the dragged payload carries a spell id, and leaves it
+    /// neutral otherwise — the handler returns 1, so the generic physical-item
+    /// fallback @ 0x004E3492 never runs for this list.
+    /// </summary>
+    internal static ItemDragAcceptance FavoriteDragOverAcceptance(object payload)
+        => payload is SpellFavoriteDragPayload or SpellbookShortcutDragPayload
+            ? ItemDragAcceptance.Accept
+            : ItemDragAcceptance.None;
+
+    /// <summary>
+    /// THE one favorite-landing computation: the index a
+    /// <see cref="SpellFavoriteDragPayload"/> drop on <paramref name="cell"/>
+    /// applies. The drag-over Accept ring and <see cref="DropFavorite"/> both key
+    /// off the same hovered cell through this method, so the ring can never
+    /// promise a different landing than the drop delivers. Two numbering spaces,
+    /// both matching retail SpellCastSubMenu::AddFavorite @ 0x004C7060:
+    /// an OCCUPIED sibling cell is still numbered against the PRE-lift bar
+    /// (Rebuild defers for the gesture — AP-172), so retail's
+    /// -1-if-removed-from-before-target adjustment applies (the
+    /// RemoveSpellFromMenu-return-gated decrement @ 0x004C7157); an EMPTY tail
+    /// cell's index clamps to the LIVE favorite count — a post-lift number whose
+    /// lifted spell is already out of the live list, exactly retail's
+    /// RemoveSpellFromMenu == -1 no-adjustment case, so applying the -1 there too
+    /// would double-correct (the off-by-one this method retired: lifting a
+    /// non-last favorite onto the empty tail landed it second-to-last instead of
+    /// last).
+    /// </summary>
+    internal int FavoriteDropIndex(
+        SpellFavoriteDragPayload payload, int targetTab, UiItemList list, UiItemSlot cell)
     {
-        // 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;
+        int index = Math.Max(0, list.IndexOf(cell));
+        if (cell.IsEmptySlot)
+            return Math.Min(index, _spellbook.GetFavorites(targetTab).Count);
+        if (payload.SourceTab == targetTab && payload.SourcePosition < index)
+            index -= 1;
+        return index;
+    }
+
+    private void DropFavorite(
+        SpellFavoriteDragPayload payload, int targetTab, UiItemList list, UiItemSlot cell)
+    {
+        int targetPosition = FavoriteDropIndex(payload, targetTab, list, cell);
         _addFavorite?.Invoke(targetTab, targetPosition, payload.SpellId);
         _selected[targetTab] = payload.SpellId;
     }