feat(vendor): Slice 6 buy arc — shop items are real objects, vendor selection is THE selection, and Buy works (0x005F)

Three ordered pieces in one landing (the shared controller/composition
files carry all three; the internal order was 6.1 -> 6.2 -> 6.3):

6.1 VendorShopItemMaterializer diff-merges the shop list into the live
ClientObjectTable on VendorState transitions (so client-local close and
session teardown retire the entries too) and never claims a guid it did
not add — ACE's UniqueItemsForSale can re-list a guid a player once
held (AP-163 files the collision-skip; no retail counterpart traced).
Right-click examine on shop items now routes through the ordinary
appraisal path — the 5.4 F7c blocker dissolves with the table entries.

6.2 SelectionChangeSource.Vendor: row clicks, auto-select, and examine
all flow through the canonical SelectionState; the status bar and the
existing byte-faithful StackSplitQuantityState slider light up
unmodified. VendorSplitPolicy is the single 0xDC41CB0 mask owner; the
slider VALUE seeds to 1 for exempt items while maxSplitSize keeps the
stack (the splitSize/maxSplitSize distinction, research §B.3).
Selection clears at retail's actual site — VendorItemsUI::RemoveFromShop
(pc:202848), not a CloseVendor-level clear that does not exist.

6.3 BuildBuy (0x005F): vendorGuid, count, (i32 amount, u32 guid) pairs,
and the trailing alternateCurrencyId the REAL client sends
(CM_Vendor::Event_Buy pc:689288) though ACE's reader ignores it.
TryBuy rides the EXISTING J5.2 one-request-at-a-time reservation and
completes on UseDone; the Buy button disables while a request is in
flight. The reconciliation round-trip (money property update, inventory
CreateObject, ApproachVendor refresh -> panel rebuild) is proven by a
synthetic-inbound test against existing machinery — no new owner.

Register: AP-161 narrowed (selection + examine residuals close;
staging/Sell remain; double-click-to-buy confirmed ABSENT from retail
with negative evidence cited — we match retail). AP-162 files the
conscious no-client-side-affordability-precheck deferral.

Clean-room complete solution: 11,368 passed / 4 skipped / 0 failed.

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

src/AcDream.App/UI/ItemInteractionController.cs

@@ -61,6 +61,8 @@ public sealed class ItemInteractionController : IDisposable
     private readonly Action<string>? _systemMessage;
     private readonly AutoWieldController _autoWield;
     private readonly Action<uint, ItemUseRequestReservation>? _requestUse;
+    // Slice 6.3: vendorGuid, itemGuid, amount, alternateCurrencyId.
+    private readonly Action<uint, uint, int, uint>? _sendBuy;
     private readonly RuntimeInteractionTransactionState _runtimeTransactions;
     private readonly InventoryTransactionState _transactions;
 
@@ -101,7 +103,8 @@ public sealed class ItemInteractionController : IDisposable
         Action<uint>? requestExternalContainer = null,
         CombatState? combatState = null,
         Action<CombatMode>? sendChangeCombatMode = null,
-        Action<uint, ItemUseRequestReservation>? requestUse = null)
+        Action<uint, ItemUseRequestReservation>? requestUse = null,
+        Action<uint, uint, int, uint>? sendBuy = null)
     {
         _objects = objects ?? throw new ArgumentNullException(nameof(objects));
         _playerGuid = playerGuid ?? throw new ArgumentNullException(nameof(playerGuid));
@@ -131,6 +134,7 @@ public sealed class ItemInteractionController : IDisposable
         _dragOnPlayerOpensSecureTrade = dragOnPlayerOpensSecureTrade ?? (() => true);
         _systemMessage = systemMessage;
         _requestUse = requestUse;
+        _sendBuy = sendBuy;
         _interactionState = interactionState
             ?? throw new ArgumentNullException(nameof(interactionState));
         _runtimeTransactions = runtimeTransactions
@@ -222,6 +226,43 @@ public sealed class ItemInteractionController : IDisposable
         return false;
     }
 
+    /// <summary>
+    /// Slice 6.3: retail's Buy button — <c>gmVendorUI::BuySingleItem</c>
+    /// (<c>pc:201661</c>, <c>0x004C2820</c>). Immediate single-item purchase
+    /// of <paramref name="itemGuid"/> (a vendor shop item) at
+    /// <paramref name="amount"/> units, with the vendor's own trade currency
+    /// (0 = pyreal). Rides the EXISTING one-request-at-a-time reservation
+    /// ordinary Use takes (research doc §A.4: every
+    /// <c>HandleActionBuyItem</c> path ends in exactly one
+    /// <c>SendUseDoneEvent()</c>, the SAME completion signal
+    /// <c>RuntimeInteractionTransactionState.CompleteUse</c> already
+    /// resolves via the wired <c>UseDone (0x01C7)</c> handler) — no second
+    /// gate, and retail's client-side busy-count increment
+    /// (<c>ClientUISystem::IncrementBusyCount</c>, <c>pc:201765</c>) is
+    /// exactly what <see cref="EnsureInventoryRequestReady"/>'s
+    /// <c>BusyCount == 0</c> check already guards for every other request.
+    /// </summary>
+    public bool TryBuy(uint vendorGuid, uint itemGuid, int amount, uint alternateCurrencyId)
+    {
+        if (vendorGuid == 0u || itemGuid == 0u || amount <= 0 || _sendBuy is null)
+            return false;
+        if (!EnsureInventoryRequestReady())
+            return false;
+
+        ItemUseRequestReservation reservation = BeginUseRequestReservation();
+        try
+        {
+            _sendBuy(vendorGuid, itemGuid, amount, alternateCurrencyId);
+        }
+        catch
+        {
+            reservation.CancelBeforeDispatch();
+            throw;
+        }
+        reservation.MarkDispatched();
+        return true;
+    }
+
     /// <summary>
     /// Retail <c>UIElement_ItemList::AcceptDragObject</c>'s local
     /// <c>m_pendingItem</c> branch. This wording belongs only to the destination

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

@@ -93,6 +93,7 @@ public sealed class SelectedObjectController : IRetainedPanelController
     private readonly Action<uint>        _sendQueryItemMana;
     private readonly StackSplitQuantityState _splitQuantity;
     private readonly SelectionState _selection;
+    private readonly Func<uint, bool> _isVendorSplitExempt;
     private readonly Action<Action<uint, float>> _unsubscribeHealthChanged;
     private readonly Action<Action<uint, float, bool>> _unsubscribeItemManaChanged;
     private readonly Action<Action<ClientObject>> _unsubscribeObjectUpdated;
@@ -126,7 +127,8 @@ public sealed class SelectedObjectController : IRetainedPanelController
         UiDatFont?          datFont,
         StackSplitQuantityState splitQuantity,
         Action<Action<ClientObject>> subscribeObjectUpdated,
-        Action<Action<ClientObject>> unsubscribeObjectUpdated)
+        Action<Action<ClientObject>> unsubscribeObjectUpdated,
+        Func<uint, bool>    isVendorSplitExempt)
     {
         _isHealthTarget  = isHealthTarget;
         _isOwnedByPlayer = isOwnedByPlayer;
@@ -139,6 +141,8 @@ public sealed class SelectedObjectController : IRetainedPanelController
         _sendQueryItemMana = sendQueryItemMana;
         _splitQuantity = splitQuantity ?? throw new ArgumentNullException(nameof(splitQuantity));
         _selection = selection ?? throw new ArgumentNullException(nameof(selection));
+        _isVendorSplitExempt = isVendorSplitExempt
+            ?? throw new ArgumentNullException(nameof(isVendorSplitExempt));
         _unsubscribeHealthChanged = unsubscribeHealthChanged;
         _unsubscribeItemManaChanged = unsubscribeItemManaChanged;
         _unsubscribeObjectUpdated = unsubscribeObjectUpdated;
@@ -250,6 +254,18 @@ public sealed class SelectedObjectController : IRetainedPanelController
     /// <param name="stackSize">Returns the stack size for a guid (0 or 1 = non-stacked).</param>
     /// <param name="sendQueryHealth">Sends retail <c>QueryHealth (0x01BF)</c>; may be a no-op offline.</param>
     /// <param name="datFont">Dat font for the name label; null = debug bitmap font fallback.</param>
+    /// <param name="isVendorSplitExempt">
+    /// Slice 6.2: retail's <c>gmToolbarUI::HandleSelectionChanged</c> vendor
+    /// branch (<c>pc:198779-198790</c>) — true when the selected guid is
+    /// owned by the currently-open vendor (its <c>ClientObject.ContainerId</c>
+    /// equals <c>VendorState.VendorId</c>) AND its type intersects
+    /// <see cref="VendorSplitPolicy.SplitExemptMask"/>. When true, a stack
+    /// seeds to quantity 1 instead of the full authored stack size — see
+    /// <see cref="ApplySelection"/>. <c>VendorUiController.VendorSplitSize</c>
+    /// answers the SAME question for vendor's own display text via the SAME
+    /// <see cref="VendorSplitPolicy"/> helper, so the mask exists in exactly
+    /// one place (composed at <c>InteractionRetainedUiComposition</c>).
+    /// </param>
     public static SelectedObjectController Bind(
         ImportedLayout layout,
         SelectionState selection,
@@ -269,14 +285,16 @@ public sealed class SelectedObjectController : IRetainedPanelController
         UiDatFont?          datFont,
         StackSplitQuantityState splitQuantity,
         Action<Action<ClientObject>> subscribeObjectUpdated,
-        Action<Action<ClientObject>> unsubscribeObjectUpdated)
+        Action<Action<ClientObject>> unsubscribeObjectUpdated,
+        Func<uint, bool>    isVendorSplitExempt)
         => new SelectedObjectController(
             layout, selection,
             subscribeHealthChanged, unsubscribeHealthChanged,
             subscribeItemManaChanged, unsubscribeItemManaChanged,
             isHealthTarget, isOwnedByPlayer, name, healthPercent, hasHealth, stackSize,
             sendQueryHealth, manaPercent, sendQueryItemMana, datFont,
-            splitQuantity, subscribeObjectUpdated, unsubscribeObjectUpdated);
+            splitQuantity, subscribeObjectUpdated, unsubscribeObjectUpdated,
+            isVendorSplitExempt);
 
     /// <summary>
     /// Port of <c>gmToolbarUI::HandleSelectionChanged</c> (<c>:198635</c>):
@@ -335,11 +353,20 @@ public sealed class SelectedObjectController : IRetainedPanelController
 
         // gmToolbarUI::HandleSelectionChanged @ 0x004BF52D..0x004BF666:
         // stacks initialize to the full stack, show the numeric entry + horizontal
-        // slider, and set the stacked selection state. Vendor-owned stack precedence
-        // is intentionally absent until the vendor panel owns an active vendor id.
+        // slider, and set the stacked selection state. Slice 6.2: the
+        // vendor-owned branch (pc:198779-198790, mask literal pc:198784)
+        // seeds splitSize (the INITIAL value) to 1 instead of the full stack
+        // when the selection is owned by the currently-open vendor AND its
+        // type intersects VendorSplitPolicy.SplitExemptMask — see the
+        // isVendorSplitExempt parameter doc. maxSplitSize (the slider's
+        // RANGE) is always the full authored stack size regardless of
+        // exemption (research doc §B.3: "Sets GenItemHolder::splitSize =
+        // seed, GenItemHolder::maxSplitSize = stackSize") — only the
+        // starting VALUE differs, not the ceiling.
         if (stackSize > 1u)
         {
-            _splitQuantity.Reset(stackSize);
+            uint seed = _isVendorSplitExempt(g) ? 1u : stackSize;
+            _splitQuantity.Reset(stackSize, initialValue: seed);
             if (_stackSizeEntry is not null) _stackSizeEntry.Visible = true;
             if (_stackSizeSlider is not null) _stackSizeSlider.Visible = true;
         }

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

@@ -5,6 +5,7 @@ using System.Linq;
 using AcDream.App.Rendering;
 using AcDream.Core.Items;
 using AcDream.Core.Properties;
+using AcDream.Core.Selection;
 
 namespace AcDream.App.UI.Layout;
 
@@ -149,19 +150,6 @@ public sealed class VendorUiController : IRetainedPanelController
     private const uint TypeMenuNormalSprite = 0x060012B3u;
     private const uint TypeMenuPressedSprite = 0x060012B4u;
 
-    /// <summary>
-    /// F2 mask rule (Slice 5.4 review): the split-exempt <c>ItemType</c>
-    /// mask <c>gmToolbarUI::HandleSelectionChanged</c> applies when seeding
-    /// <c>GenItemHolder::splitSize</c> for a vendor-owned selection
-    /// (<c>pc:198779-198790</c>, literal mask at <c>pc:198784</c>). Every
-    /// row <see cref="VendorUiController"/> shows IS vendor-owned (its
-    /// container is unconditionally the open vendor), so the "does this
-    /// item belong to the open vendor" gate that precedes the mask check in
-    /// retail's function is always true here and is not reproduced
-    /// separately — see <see cref="VendorSplitSize"/>.
-    /// </summary>
-    private const uint SplitExemptMask = 0x0DC41CB0u;
-
     /// <summary>
     /// Retail's ordered category table, transcribed verbatim from
     /// <c>VendorItemsUI::OpenVendor</c>'s <c>AddTypeFilter</c> call chain
@@ -197,6 +185,9 @@ public sealed class VendorUiController : IRetainedPanelController
     private readonly Func<ItemType, uint, uint, uint, uint, uint> _resolveIcon;
     private readonly ClientObjectTable _objects;
     private readonly Func<uint> _playerGuid;
+    private readonly ItemInteractionController _itemInteraction;
+    private readonly SelectionState _selection;
+    private readonly StackSplitQuantityState _splitQuantity;
     private readonly UiElement _itemsPage;
     private readonly UiElement _buyingPage;
     private readonly UiElement _sellingPage;
@@ -213,7 +204,11 @@ public sealed class VendorUiController : IRetainedPanelController
 
     private readonly List<(string Label, ItemType Mask)> _presentCategories = new();
     private int _selectedCategoryIndex = -1;
-    private uint _selectedItemGuid;
+    // Slice 6.3: tracks "does the CURRENT selection permit Buy/Add" separately
+    // from "is a Buy request currently in flight" (RecomputeBuyButtonEnabled
+    // combines both for the Buy button specifically; Add is selection-only,
+    // per contract decision 6 — staging stays unwired this pass).
+    private bool _buyEnabledBySelection;
     private bool _disposed;
 
     private VendorUiController(
@@ -222,6 +217,9 @@ public sealed class VendorUiController : IRetainedPanelController
         Func<ItemType, uint, uint, uint, uint, uint> resolveIcon,
         ClientObjectTable objects,
         Func<uint> playerGuid,
+        ItemInteractionController itemInteraction,
+        SelectionState selection,
+        StackSplitQuantityState splitQuantity,
         UiElement itemsPage,
         UiElement buyingPage,
         UiElement sellingPage,
@@ -246,6 +244,9 @@ public sealed class VendorUiController : IRetainedPanelController
         _resolveIcon = resolveIcon;
         _objects = objects;
         _playerGuid = playerGuid;
+        _itemInteraction = itemInteraction;
+        _selection = selection;
+        _splitQuantity = splitQuantity;
         _itemsPage = itemsPage;
         _buyingPage = buyingPage;
         _sellingPage = sellingPage;
@@ -276,6 +277,13 @@ public sealed class VendorUiController : IRetainedPanelController
         {
             SpriteResolve = _itemList.SpriteResolve,
         };
+        // Slice 6.1: mirrors ExternalContainerController's own
+        // right-click-examine wiring (UiItemSlot.OnEvent's RightClick case).
+        // Now that shop items are materialized into ClientObjectTable (see
+        // VendorShopItemMaterializer), AppraisalUiController.Apply's lookup
+        // succeeds and this stops being a dead end — closes half of AP-161
+        // finding #2.
+        _itemList.ExamineItemRequested = ExamineItem;
         if (itemScrollbar is not null)
         {
             itemScrollbar.Model = _itemList.Scroll;
@@ -310,11 +318,40 @@ public sealed class VendorUiController : IRetainedPanelController
         RetailTabBinding.SetClick(_sellingTab, () => ShowTab(VendorPanelTab.Selling));
         if (_close is not null)
             _close.OnClick = () => _vendor.Close();
+        // Slice 6.3: retail gmVendorUI::HandleButtonClicks' 0x100000C2 case —
+        // BuySingleItem(selectedID) — an immediate single-item purchase, no
+        // staging list required (research doc §B.1).
+        if (_buyButton is not null)
+            _buyButton.OnClick = BuySelectedItem;
 
         ShowTab(VendorPanelTab.Items);
         ClearContent();
 
         _vendor.Changed += OnVendorChanged;
+        // Slice 6.2: SelectionState is now the AUTHORITY (research doc §B.4:
+        // vendor-context selections flow through the SAME global
+        // ACCWeenieObject::SetSelectedObject primitive as every other
+        // origin) — this panel is a CONSUMER, mirroring
+        // ExternalContainerController.OnSelectionChanged's shape exactly.
+        _selection.Changed += OnSelectionTransition;
+        // Slice 6.2: mirrors ExternalContainerController.OnObjectRemoved —
+        // retail's VendorItemsUI::RemoveFromShop (pc:202848-202850,
+        // 0x004c3d4a) clears the global selection when a shop item leaves
+        // the list (there is no separate CloseVendor-level SetSelectedObject(0)
+        // call; retail's ItemList_Flush on close does not touch selection
+        // directly). Since VendorShopItemMaterializer removes every
+        // materialized item from ClientObjectTable on Close/Reset/replace
+        // (Slice 6.1), subscribing here gives "vendor session close clears
+        // a vendor-owned selection" as a consequence of the SAME generic
+        // mechanism every other panel already uses, not a vendor-specific
+        // special case.
+        _objects.ObjectRemoved += OnObjectRemoved;
+        // Slice 6.3: mirrors ExternalContainerController's own
+        // _itemInteraction.StateChanged subscription — the Buy button must
+        // disable the instant a reservation is taken (BeginUseRequestReservation
+        // increments BusyCount synchronously, before the wire send), and
+        // re-enable on the matching UseDone/cancel, without polling.
+        _itemInteraction.StateChanged += OnInteractionStateChanged;
     }
 
     /// <param name="objects">
@@ -325,6 +362,28 @@ public sealed class VendorUiController : IRetainedPanelController
     /// <c>ObjectTableWiring</c>'s <c>PrivateUpdatePropertyInt</c> routing.
     /// </param>
     /// <param name="playerGuid">Resolves the local player's guid to look up in <paramref name="objects"/>.</param>
+    /// <param name="itemInteraction">
+    /// Slice 6.1: the shared retail item interaction orchestrator — its
+    /// <c>ExamineSelectedOrEnterMode</c> is what right-click-examine on a
+    /// shop row routes through, mirroring
+    /// <see cref="ExternalContainerController"/>'s own examine wiring.
+    /// </param>
+    /// <param name="selection">
+    /// Slice 6.2: the canonical <see cref="SelectionState"/> — now the
+    /// AUTHORITY for shop-row selection (row clicks, the F4 auto-select
+    /// fallback, and right-click examine all call
+    /// <see cref="SelectionState.Select"/> directly); this panel only
+    /// listens and reacts, the same way every sibling panel does.
+    /// </param>
+    /// <param name="splitQuantity">
+    /// Slice 6.3: the shared toolbar stack-quantity control (retail
+    /// <c>GenItemHolder::splitSize</c>/<c>maxSplitSize</c>) — the Buy button
+    /// reads the SAME live value <see cref="SelectedObjectController"/>
+    /// seeds/the player adjusts via <c>ItemHolder::GetObjectSplitSize</c>
+    /// (<c>0x00586F00</c>), matching retail's <c>BuySingleItem</c>
+    /// (<c>pc:201674-201681</c>: quantity 1 if <c>_stackSize &lt;= 1</c>,
+    /// else the current slider value).
+    /// </param>
     /// <param name="datFont">Retail dat font for the category dropdown's button/row labels.</param>
     /// <param name="debugFont">Fallback debug bitmap font (used when <paramref name="datFont"/> is null).</param>
     /// <param name="resolveSprite">Dat RenderSurface id → (GL tex handle, px width, px height).</param>
@@ -336,6 +395,9 @@ public sealed class VendorUiController : IRetainedPanelController
         Func<ItemType, uint, uint, uint, uint, uint> resolveIcon,
         ClientObjectTable objects,
         Func<uint> playerGuid,
+        ItemInteractionController itemInteraction,
+        SelectionState selection,
+        StackSplitQuantityState splitQuantity,
         UiDatFont? datFont,
         BitmapFont? debugFont,
         Func<uint, (uint tex, int w, int h)> resolveSprite,
@@ -347,6 +409,9 @@ public sealed class VendorUiController : IRetainedPanelController
         ArgumentNullException.ThrowIfNull(resolveIcon);
         ArgumentNullException.ThrowIfNull(objects);
         ArgumentNullException.ThrowIfNull(playerGuid);
+        ArgumentNullException.ThrowIfNull(itemInteraction);
+        ArgumentNullException.ThrowIfNull(selection);
+        ArgumentNullException.ThrowIfNull(splitQuantity);
         ArgumentNullException.ThrowIfNull(resolveSprite);
 
         if (layout.FindElement(ItemsPageId) is not { } itemsPage
@@ -374,6 +439,9 @@ public sealed class VendorUiController : IRetainedPanelController
             resolveIcon,
             objects,
             playerGuid,
+            itemInteraction,
+            selection,
+            splitQuantity,
             itemsPage,
             buyingPage,
             sellingPage,
@@ -517,11 +585,12 @@ public sealed class VendorUiController : IRetainedPanelController
     /// FIRST item that passed the filter becomes the display selection when
     /// the previous one didn't survive it, and the list unconditionally
     /// scrolls back to its start (<c>ScrollToShow(m_shopList, 0)</c>).
-    /// Retail routes the selection through the global
-    /// <c>ACCWeenieObject::selectedID</c>/<c>SetSelectedObject</c>; this
-    /// keeps the existing PRIVATE <see cref="_selectedItemGuid"/> selection
-    /// instead of wiring that global seam (deferred — see the register,
-    /// AP-161).
+    /// Slice 6.2: retail routes the selection through the global
+    /// <c>ACCWeenieObject::selectedID</c>/<c>SetSelectedObject</c>
+    /// (<c>pc:201184</c>, confirmed to be the SAME primitive as the fallback
+    /// select here) — this now calls <see cref="SelectionState.Select"/>
+    /// instead of the retired private field, so the toolbar status bar and
+    /// slider light up for the auto-selected item too.
     /// </remarks>
     private void RebuildItemList()
     {
@@ -531,6 +600,7 @@ public sealed class VendorUiController : IRetainedPanelController
         uint maskValue = (uint)activeMask;
 
         IReadOnlyList<VendorShopItem> items = _vendor.Items;
+        uint? selectedGuid = _selection.SelectedObjectId;
         bool selectionStillPresent = false;
         VendorShopItem? firstItem = null;
 
@@ -544,7 +614,7 @@ public sealed class VendorUiController : IRetainedPanelController
                     if (((item.ItemType ?? 0u) & maskValue) == 0u) continue;
 
                     firstItem ??= item;
-                    if (item.ItemGuid == _selectedItemGuid) selectionStillPresent = true;
+                    if (item.ItemGuid == selectedGuid) selectionStillPresent = true;
 
                     // F5 (Slice 5.4 review): forward the icon underlay/
                     // overlay/effects PublicWeenieDescParser already
@@ -562,9 +632,9 @@ public sealed class VendorUiController : IRetainedPanelController
                         SlotIndex = _itemList.GetNumUIItems(),
                     };
                     cell.SetItem(item.ItemGuid, icon);
-                    cell.Selected = item.ItemGuid == _selectedItemGuid;
+                    cell.Selected = item.ItemGuid == selectedGuid;
                     VendorShopItem captured = item;
-                    cell.Clicked = () => SelectItem(captured);
+                    cell.Clicked = () => _selection.Select(captured.ItemGuid, SelectionChangeSource.Vendor);
                     _itemList.AddItem(cell);
                 }
             }
@@ -572,8 +642,10 @@ public sealed class VendorUiController : IRetainedPanelController
 
         if (!selectionStillPresent)
         {
-            if (firstItem is { } first) SelectItem(first);
-            else ClearSelection();
+            if (firstItem is { } first)
+                _selection.Select(first.ItemGuid, SelectionChangeSource.Vendor);
+            else
+                _selection.Clear(SelectionChangeSource.Vendor);
         }
 
         // F7a: unconditional scroll-to-start on every rebuild (retail only
@@ -585,7 +657,13 @@ public sealed class VendorUiController : IRetainedPanelController
     /// <summary>
     /// Port of retail row selection display —
     /// <c>VendorItemsUI::UpdateItemsUI</c> (<c>0x004C38E0</c>,
-    /// <c>pc:202539-202820</c>).
+    /// <c>pc:202539-202820</c>). Slice 6.2: called ONLY from
+    /// <see cref="OnSelectionTransition"/>, once <paramref name="item"/> has
+    /// already been confirmed to be the globally-selected guid — this method
+    /// no longer writes the selection itself (<see cref="SelectionState"/>
+    /// is the authority; row clicks, the F4 auto-select fallback, and
+    /// right-click examine all call <see cref="SelectionState.Select"/>
+    /// directly and let this method react).
     /// </summary>
     /// <remarks>
     /// F2/F3 (Slice 5.4 review): the priced/named QUANTITY is retail's
@@ -608,14 +686,12 @@ public sealed class VendorUiController : IRetainedPanelController
     /// retail state pair (see <c>ToolbarController</c>'s
     /// <c>_useButton.Enabled</c>).
     /// </remarks>
-    private void SelectItem(VendorShopItem item)
+    private void ApplyItemDisplay(VendorShopItem item)
     {
-        _selectedItemGuid = item.ItemGuid;
-
         for (int i = 0; i < _itemList.GetNumUIItems(); i++)
         {
             if (_itemList.GetItem(i) is { } cell)
-                cell.Selected = cell.ItemId == _selectedItemGuid;
+                cell.Selected = cell.ItemId == item.ItemGuid;
         }
 
         int quantity = VendorSplitSize(item);
@@ -635,20 +711,86 @@ public sealed class VendorUiController : IRetainedPanelController
         SetActionButtonsEnabled(true);
     }
 
+    /// <summary>
+    /// Right-click examine on a shop row — mirrors
+    /// <c>ExternalContainerController.ExamineItem</c>'s "select then
+    /// request appraisal" shape, matching retail's single-selection model
+    /// (research doc §B.2: no dedicated double-click mechanism, plain
+    /// select-then-act). Slice 6.2: routes through
+    /// <see cref="SelectionState.Select"/> — <see cref="OnSelectionTransition"/>
+    /// applies the display update, so this method no longer needs to search
+    /// <see cref="_vendor"/>'s items itself.
+    /// </summary>
+    private void ExamineItem(uint guid)
+    {
+        _selection.Select(guid, SelectionChangeSource.Vendor);
+        _itemInteraction.ExamineSelectedOrEnterMode(guid);
+    }
+
+    /// <summary>
+    /// Slice 6.2: reacts to ANY global selection change, not just ones this
+    /// panel originated — mirrors <c>ExternalContainerController.OnSelectionChanged</c>.
+    /// Shows this panel's own price/name text for the newly-selected guid
+    /// when it is one of <see cref="_vendor"/>'s current items; clears the
+    /// panel's display otherwise (a selection made in some OTHER panel while
+    /// the vendor window is open must not leave stale vendor pricing text
+    /// on screen).
+    /// </summary>
+    private void OnSelectionTransition(SelectionTransition transition)
+    {
+        _ = transition;
+        uint? selected = _selection.SelectedObjectId;
+        if (selected is { } guid)
+        {
+            foreach (VendorShopItem item in _vendor.Items)
+            {
+                if (item.ItemGuid == guid)
+                {
+                    ApplyItemDisplay(item);
+                    return;
+                }
+            }
+        }
+        ClearSelectionDisplay();
+    }
+
+    /// <summary>
+    /// Slice 6.2: retail's <c>VendorItemsUI::RemoveFromShop</c>
+    /// (<c>pc:202848-202850</c>, <c>0x004c3d4a</c>) clears the global
+    /// selection when a shop item leaves the vendor's list — this mirrors
+    /// that (and, transitively, every other panel's own
+    /// <c>OnObjectRemoved</c>) rather than a vendor-specific "on close, set
+    /// selected to 0" special case. <c>VendorShopItemMaterializer</c>
+    /// removing every materialized item on session close/reset/replace
+    /// (Slice 6.1) is therefore what actually drives "vendor session close
+    /// clears a vendor-owned selection."
+    /// </summary>
+    private void OnObjectRemoved(ClientObject item)
+    {
+        if (_selection.SelectedObjectId == item.ObjectId)
+        {
+            _selection.Clear(
+                SelectionChangeSource.Vendor,
+                SelectionChangeReason.SelectedObjectRemoved);
+        }
+    }
+
     /// <summary>
     /// The quantity retail prices/names a vendor-shop selection at —
     /// <c>gmToolbarUI::HandleSelectionChanged</c>'s vendor-owned branch
-    /// (<c>pc:198779-198790</c>, mask literal at <c>pc:198784</c>): 1 unit
-    /// if the item's type intersects <see cref="SplitExemptMask"/>, else its
-    /// own authored stack size (0/absent treated as 1, matching
-    /// <c>ItemHolder::GetObjectSplitSize</c>'s own <c>stackSize==0 -&gt; 1</c>
-    /// floor, <c>pc:401473-401476</c>).
+    /// (<c>pc:198779-198790</c>). Every row <see cref="VendorUiController"/>
+    /// shows IS vendor-owned (its container is unconditionally the open
+    /// vendor), so the "does this item belong to the open vendor" gate that
+    /// precedes the mask check in retail's function is always true here and
+    /// is not reproduced separately. Slice 6.2: delegates to
+    /// <see cref="VendorSplitPolicy"/> — the single source of truth for the
+    /// <c>0xDC41CB0</c> mask, also used by <c>SelectedObjectController</c>'s
+    /// REAL seeding path (<c>InteractionRetainedUiComposition</c>'s
+    /// <c>isVendorSplitExempt</c> delegate) so the mask exists in exactly
+    /// one place.
     /// </summary>
-    private static int VendorSplitSize(VendorShopItem item)
-    {
-        if (((item.ItemType ?? 0u) & SplitExemptMask) != 0u) return 1;
-        return item.DescStackSize is { } size && size > 0 ? size : 1;
-    }
+    private static int VendorSplitSize(VendorShopItem item) =>
+        VendorSplitPolicy.SeedQuantity((ItemType)(item.ItemType ?? 0u), item.DescStackSize);
 
     /// <summary>
     /// Cost sentence — <c>VendorItemsUI::UpdateItemsUI</c>'s tail
@@ -698,9 +840,8 @@ public sealed class VendorUiController : IRetainedPanelController
             playerTotal.ToString("N0", CultureInfo.InvariantCulture));
     }
 
-    private void ClearSelection()
+    private void ClearSelectionDisplay()
     {
-        _selectedItemGuid = 0u;
         for (int i = 0; i < _itemList.GetNumUIItems(); i++)
         {
             if (_itemList.GetItem(i) is { } cell)
@@ -713,8 +854,65 @@ public sealed class VendorUiController : IRetainedPanelController
 
     private void SetActionButtonsEnabled(bool enabled)
     {
-        if (_buyButton is not null) _buyButton.Enabled = enabled;
+        _buyEnabledBySelection = enabled;
         if (_addButton is not null) _addButton.Enabled = enabled;
+        RecomputeBuyButtonEnabled();
+    }
+
+    /// <summary>
+    /// Slice 6.3: the Buy button's enabled state is the CONJUNCTION of "is
+    /// something selected" (<see cref="_buyEnabledBySelection"/>, set by
+    /// <see cref="SetActionButtonsEnabled"/>) and "is the shared inventory/
+    /// use gate free right now" (<see cref="ItemInteractionController.CanMakeInventoryRequest"/>).
+    /// The Add button (staging, contract decision 6 — unwired this pass)
+    /// stays selection-only. Called on every selection change AND on every
+    /// <see cref="ItemInteractionController.StateChanged"/> tick, so the
+    /// button disables the instant <see cref="ItemInteractionController.TryBuy"/>
+    /// takes its reservation and re-enables on the matching completion —
+    /// no per-frame polling.
+    /// </summary>
+    private void RecomputeBuyButtonEnabled()
+    {
+        if (_buyButton is not null)
+            _buyButton.Enabled = _buyEnabledBySelection && _itemInteraction.CanMakeInventoryRequest;
+    }
+
+    private void OnInteractionStateChanged() => RecomputeBuyButtonEnabled();
+
+    /// <summary>
+    /// Slice 6.3: retail <c>gmVendorUI::BuySingleItem</c> (<c>pc:201661</c>).
+    /// Reads the CURRENT globally-selected shop item and the CURRENT split
+    /// quantity, then dispatches a single-item purchase through the shared
+    /// use/inventory reservation. Client-side affordability/capacity
+    /// pre-checks are deliberately NOT ported (research doc's open question
+    /// 1: the server is authoritative either way and pre-checks are latency/
+    /// UX polish, not correctness — deferred as a fast follow-up if the
+    /// round-trip lag on a refused purchase is noticeable live).
+    /// </summary>
+    private void BuySelectedItem()
+    {
+        if (_selection.SelectedObjectId is not { } guid)
+            return;
+
+        VendorShopItem? selected = null;
+        foreach (VendorShopItem item in _vendor.Items)
+        {
+            if (item.ItemGuid == guid)
+            {
+                selected = item;
+                break;
+            }
+        }
+        if (selected is not { } shopItem)
+            return;
+
+        uint stackSize = (uint)Math.Max(shopItem.DescStackSize ?? 1, 1);
+        uint quantity = _splitQuantity.GetObjectSplitSize(shopItem.ItemGuid, guid, stackSize);
+        _itemInteraction.TryBuy(
+            _vendor.VendorId,
+            shopItem.ItemGuid,
+            (int)quantity,
+            _vendor.Profile.AlternateCurrencyWcid);
     }
 
     private void ClearContent()
@@ -724,7 +922,13 @@ public sealed class VendorUiController : IRetainedPanelController
         _typeMenu.Items = Array.Empty<UiMenu.MenuItem>();
         _typeMenu.Selected = null;
         _itemList.Flush();
-        ClearSelection();
+        // Local widget hygiene only — does NOT touch the global selection.
+        // This runs from the constructor (before any vendor is ever open)
+        // and cannot assume whatever SelectionState.SelectedObjectId
+        // currently holds belongs to this panel. The actual "vendor session
+        // close clears a vendor-owned selection" behavior is OnObjectRemoved
+        // reacting to VendorShopItemMaterializer's removal, not this method.
+        ClearSelectionDisplay();
     }
 
     private static void SetPlainText(UiText text, string value)
@@ -740,12 +944,18 @@ public sealed class VendorUiController : IRetainedPanelController
         if (_disposed) return;
         _disposed = true;
         _vendor.Changed -= OnVendorChanged;
+        _selection.Changed -= OnSelectionTransition;
+        _objects.ObjectRemoved -= OnObjectRemoved;
+        _itemInteraction.StateChanged -= OnInteractionStateChanged;
         RetailTabBinding.SetClick(_itemsTab, null);
         RetailTabBinding.SetClick(_buyingTab, null);
         RetailTabBinding.SetClick(_sellingTab, null);
         _typeMenu.OnSelect = null;
         _typeMenu.ButtonLabelProvider = null;
+        _itemList.ExamineItemRequested = null;
         if (_close is not null)
             _close.OnClick = null;
+        if (_buyButton is not null)
+            _buyButton.OnClick = null;
     }
 }

src/AcDream.App/UI/RetailUiRuntime.cs

@@ -112,7 +112,11 @@ public sealed record ToolbarRuntimeBindings(
     Action<uint> SendQueryHealth,
     Action<uint> SendQueryItemMana,
     Func<uint> PlayerGuid,
-    Action<uint, uint, int>? SendPutItemInContainer);
+    Action<uint, uint, int>? SendPutItemInContainer,
+    // Slice 6.2: composed at InteractionRetainedUiComposition from
+    // d.Inventory.Vendor + d.Inventory.Objects — SelectedObjectController's
+    // vendor-owned split-exempt-seed predicate (research doc §C.1).
+    Func<uint, bool> IsVendorSplitExempt);
 
 public sealed record CharacterRuntimeBindings(CharacterSheetProvider Provider);
 
@@ -170,7 +174,15 @@ public sealed record AppraisalRuntimeBindings(
 
 public sealed record VendorRuntimeBindings(
     VendorState State,
-    Func<ItemType, uint, uint, uint, uint, uint> ResolveIcon);
+    Func<ItemType, uint, uint, uint, uint, uint> ResolveIcon,
+    // Slice 6.1: shared examine routing — VendorUiController.ExamineItem
+    // calls ItemInteraction.ExamineSelectedOrEnterMode the same way
+    // ExternalContainerRuntimeBindings.ItemInteraction's consumer does.
+    ItemInteractionController ItemInteraction,
+    // Slice 6.2: the canonical selection authority — every sibling binding
+    // record (Radar/Magic/Toolbar/Inventory/ExternalContainer) already
+    // carries this; Vendor was the one outlier (research doc §C.1).
+    SelectionState Selection);
 
 public sealed record RetailUiRuntimeBindings(
     UiHost Host,
@@ -763,7 +775,8 @@ public sealed class RetailUiRuntime : IDisposable
             _bindings.Assets.DefaultFont,
             StackSplitQuantity,
             handler => b.Objects.ObjectUpdated += handler,
-            handler => b.Objects.ObjectUpdated -= handler);
+            handler => b.Objects.ObjectUpdated -= handler,
+            b.IsVendorSplitExempt);
 
         UiElement root = layout.Root;
         RetailWindowHandle handle = RetailWindowFrame.Mount(
@@ -1994,6 +2007,9 @@ public sealed class RetailUiRuntime : IDisposable
             b.ResolveIcon,
             _bindings.Inventory.Objects,
             _bindings.Inventory.PlayerGuid,
+            b.ItemInteraction,
+            b.Selection,
+            StackSplitQuantity,
             _bindings.Assets.DefaultFont,
             _bindings.Assets.DebugFont,
             _bindings.Assets.ResolveSprite,