Potion counts on the bottom bar and potion stacks in the inventory

- potioncounts.cpp: each potion slot shows how many potions of its kind the
  hero carries (stacks added up)
- potionstack.cpp: potions of one type stack up to 20 per cell; every
  potion entering the hero's grid pours into stacks with room, the sort
  button stacks first, drinking takes one off a stack, counts drawn on the
  cells and saved in the JSON inventories
- game/engine/items.h: potion kinds and grid helpers; ObjectManager and
  DestroyObject in objects.h
- auto-attack: the projectile-weapon target radius is a design value
- docs/potion-stacks.md: measured paths and the design

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -43,6 +43,8 @@ add_library(sacred_re SHARED
     src/mods/input.cpp
     src/mods/skillslots.cpp
     src/mods/inventory_save.cpp
+    src/mods/potioncounts.cpp
+    src/mods/potionstack.cpp
     src/mods/skillbar.cpp
     src/mods/arttarget.cpp
     src/mods/aura_restore.cpp

docs/potion-stacks.md

@@ -0,0 +1,59 @@
+# Potion counts and potion stacks
+
+## Requests (2026-10-17)
+
+1. The potion selector on the bottom bar always shows how many potions of
+   each kind the hero carries (only when there are any).
+2. Potions of the same type stack in one inventory cell, up to 20 per stack.
+   Every potion entering the inventory tries to stack. Drag, sell and drop
+   move a whole stack. The inventory's sort button stacks first, then sorts.
+   A stack dropped on the ground splits into single potions (agreed for now).
+
+## Measured (diagnostic log, 2026-10-17)
+
+| Action | Calls |
+|---|---|
+| Drink (potion key) | `cEngine::UseActionSlot` `0x613040` → `cCreature::UseItem` `0x54a8f0` → `RemoveItem` (from `0x54aa20`) → `DestroyObject` (from `0x54aa30`); then `UseActionSlot` calls `RemoveItem` again (`0x6130d8`, not found) |
+| Drag in the inventory | `RemoveAt` from `0x6c06fb` (cell → cursor), `PlaceItem` from `0x6c0583` (cursor → cell) |
+| Buy | trader grid (inventory 22) `RemoveItem` `0x6d96f5` → hero grid `PlaceItem` `0x6d98e8` |
+| Pick up from the ground | `PlaceItem` from `0x558db6` |
+| Trader restock | `RemoveItem` `0x6d9973` + `DestroyObject` `0x6d99cc`, new items `PlaceItem` `0x6d9bdf` |
+| Sort | `cInventoryEntry::Sort` `0x5db670`: all items removed (`0x5db8a5`) and placed again (`0x5dbb4c`, notify 0) |
+
+Not seen yet: a sale, a right-click drink.
+
+## Game side
+
+- Potion kinds: `ItemPotionKind` `0x43f630` (ecx = item type table
+  `*0xaa9564`) maps an item type to kind 1..5 (the bottom bar's potion
+  slots); `GetPotionRef` `0x5da880` picks the potion a slot drinks.
+- Bottom bar: `cUI_Taskbar2::RenderPotionSlots` `0x6e39e0` draws the 5 slot
+  controls (embedded at +0x258, stride 0xbc).
+- Inventory grid (`cInventoryEntry`, docs/skill-slots.md): cells of 0xc bytes
+  at +0x18; an item's id only in its first cell, covered cells flagged.
+  `CellRect` `0x5dd530` gives a cell's screen rect (32x32 from the origin
+  +0x1164 / +0x1166, which the window sets before `DrawItems` `0x5dc3f0`).
+- Drinking: `cPotion3D::vf18` `0x5ca9d0` applies the effect and sets object
+  flag +0x14 bit `0x8000000` (used up); `UseItem` then removes and destroys.
+- Sort button: `cUI_Inventory3::HandleEvent` `0x6c5f40`, button control at
+  +0x158, mode from bit 4 of +0x168.
+- Hero files map saved item ids to new objects with `ReplaceItemId`
+  `0x5d80e0`.
+
+## Mods
+
+- `src/mods/potioncounts.cpp`: counts per kind (stacks added up), drawn
+  bottom-right on each potion slot after `RenderPotionSlots`.
+- `src/mods/potionstack.cpp`: stack counts by item id (with the item type),
+  saved as `"count"` on the grid cell in the JSON inventories
+  (`inventory_save.cpp`); entry pours into stacks with room on the next
+  tick; `Sort` stacks the hero's potions first; a drink from a stack of more
+  than one takes one off the count and keeps the object; the count is drawn
+  on the cell after `DrawItems`.
+
+## Open
+
+- Selling a stack: the price is for one potion so far (sale path not measured).
+- Dropping a stack on the ground: it should split into single potions; for now
+  the count stays with the object while the game runs, but a save keeps only
+  the object (one potion).

docs/todo.md

@@ -5,8 +5,8 @@
 Saving marks effect objects (cObjectFX) dead; loading keeps the buff but drops
 the aura effect (type 842) that follows the hero. `src/mods/aura_restore.cpp`
 recreates it after a load for Heldenmut (0x3f7) and Klingenblick (0x3f8). Spell
-auras (Steinhaut, Lichtschild, ...) use other effects and are not covered
-yet; check them if their visuals go missing too.
+auras (Steinhaut, Lichtschild, ...) use other effects; the user confirmed on
+2026-10-17 that their visuals are back after loading too.
 
 ## Data-driven arts instead of hardcoded art ids
 
@@ -125,3 +125,12 @@ in-game texts/tooltips that name the old keys):
 
 Globally bound letters (engine `0x618130`, UI manager `0x757120`): A B C E F
 H I L M N O P Q R S T W, Space, ^, digits. Individual windows may use more.
+
+## Potion stacks
+
+Asked 2026-10-17 (after the potion counts on the bottom bar,
+`src/mods/potioncounts.cpp`): potions of the same type (e.g. minor healing)
+stack in one inventory cell, up to 20 per stack (a full stack starts a new
+one). Drag, sell and drop move the whole stack, as with any other item. The
+counts on the potion slots then add up the stacks. In progress, see
+docs/potion-stacks.md (open: sale price per stack, splitting on the ground).

src/game/engine/fx.cpp

@@ -2,6 +2,7 @@
 
 #include <vector>
 
+#include "game/engine/objects.h"
 #include "re/hook.h"
 
 namespace game {
@@ -29,7 +30,7 @@ void BeforeFxEvent(FxEventFn callback) {
 
 uint32_t CreateObjectAt(uint32_t type, const void* position, uint32_t link) {
     using Fn = uint32_t(__fastcall*)(void* manager, void* edx, uint32_t, const void*, uint32_t, uint32_t, uint32_t);
-    void* manager = *reinterpret_cast<void**>(0x00ad3bc0);
+    void* manager = ObjectManager();
     return manager ? reinterpret_cast<Fn>(0x005fbaa0)(manager, nullptr, type, position, link, 0, 0) : 0;
 }
 

src/game/engine/items.h

@@ -0,0 +1,67 @@
+#pragma once
+// Items and the inventory grid (cInventoryEntry, docs/skill-slots.md).
+
+#include <cstddef>
+#include <cstdint>
+
+#include "game/engine/objects.h"
+
+namespace game {
+
+// Item objects (cItemBase and subclasses such as cPotion3D).
+constexpr size_t kItemType = 0x10;
+constexpr uint32_t kObjectUsedUp = 0x8000000;  // kObjectFlags: set by cPotion3D::vf18 when drunk
+
+// The item type table at *0xaa9564 (ecx of ItemPotionKind).
+constexpr uint32_t kItemTypeTable = 0x00aa9564;
+
+// ItemPotionKind (0x43f630): the potion kind (1..5, the bottom bar's potion
+// slots; 1 = healing) of item type `type`, 0 if it is no potion.
+constexpr int kPotionKinds = 5;
+inline int ItemPotionKind(uint32_t type) {
+    using Fn = int(__fastcall*)(void* table, void* edx, uint32_t type);
+    void* table = *reinterpret_cast<void**>(kItemTypeTable);
+    return table ? reinterpret_cast<Fn>(0x0043f630)(table, nullptr, type) : 0;
+}
+
+inline uint32_t ItemType(const void* item) {
+    return *reinterpret_cast<const uint32_t*>(static_cast<const uint8_t*>(item) + kItemType);
+}
+
+// The potion kind of object `id`, 0 if it is no potion (or gone).
+inline int PotionKindOf(uint32_t id) {
+    const void* item = id ? GetObjectById(id) : nullptr;
+    return item ? ItemPotionKind(ItemType(item)) : 0;
+}
+
+// cInventoryEntry item grid: cells of {u32 item id, u8 flags, u8, u8, u8,
+// u16 type, u16}. An item's id is in its first cell only; the other cells it
+// covers have id 0, flags bit 0 and the first cell's x, y at +5 / +6
+// (cInventoryEntry::GetItemAt 0x5da290 resolves them).
+constexpr size_t kInventoryGrid = 0x18;
+constexpr size_t kGridCellSize = 0xc;
+constexpr size_t kGridWidth = 0x115c;   // u16, 0x14
+constexpr size_t kGridHeight = 0x115e;  // u16, 8
+constexpr size_t kInventoryOwner = 0x1160;  // u16: player index (heroes < 0x12)
+
+inline unsigned GridCells(const uint8_t* inventory) {
+    return *reinterpret_cast<const uint16_t*>(inventory + kGridWidth) *
+           *reinterpret_cast<const uint16_t*>(inventory + kGridHeight);
+}
+
+inline uint32_t GridItem(const uint8_t* inventory, unsigned cell) {
+    return *reinterpret_cast<const uint32_t*>(inventory + kInventoryGrid + cell * kGridCellSize);
+}
+
+// Calls `fn(id)` for each item in `inventory`'s grid.
+template <typename Fn>
+void ForEachGridItem(const uint8_t* inventory, Fn fn) {
+    const unsigned cells = GridCells(inventory);
+    for (unsigned cell = 0; cell < cells; ++cell) {
+        if (const uint32_t id = GridItem(inventory, cell)) {
+            fn(id);
+        }
+    }
+}
+
+} // namespace game

src/game/engine/objects.h

@@ -38,13 +38,27 @@ inline void* DynamicCast(void* object, uint32_t sourceType, uint32_t targetType)
     return object ? reinterpret_cast<Fn>(0x00849701)(object, 0, sourceType, targetType, 0) : nullptr;
 }
 
-// cObjectManager::GetObject (0x5fe060) on the manager at *0xad3bc0.
+// The object manager (cObjectManager at *0xad3bc0), or null.
+inline void* ObjectManager() {
+    return *reinterpret_cast<void**>(0x00ad3bc0);
+}
+
+// cObjectManager::GetObject (0x5fe060).
 inline void* GetObjectById(uint32_t id) {
     using Fn = void*(__fastcall*)(void* manager, void* edx, uint32_t id);
-    void* manager = *reinterpret_cast<void**>(0x00ad3bc0);
+    void* manager = ObjectManager();
     return manager ? reinterpret_cast<Fn>(0x005fe060)(manager, nullptr, id) : nullptr;
 }
 
+// cObjectManager::DestroyObject (0x5fbe10), with the arguments cCreature::UseItem
+// destroys a drunk potion with (id, 1, 0, 0).
+inline void DestroyObject(uint32_t id) {
+    using Fn = uint32_t(__fastcall*)(void* manager, void* edx, uint32_t, uint32_t, uint32_t, uint32_t);
+    if (void* manager = ObjectManager()) {
+        reinterpret_cast<Fn>(0x005fbe10)(manager, nullptr, id, 1, 0, 0);
+    }
+}
+
 // The object `id` as a cCreature, or null.
 inline void* GetCreature(uint32_t id) {
     return DynamicCast(GetObjectById(id), kTypeObject, kTypeCreature);
@@ -54,7 +68,7 @@ inline void* GetCreature(uint32_t id) {
 // cCreature), or null.
 inline void* GetPlayerHero() {
     using Fn = void*(__fastcall*)(void* manager, void* edx);
-    void* manager = *reinterpret_cast<void**>(0x00ad3bc0);
+    void* manager = ObjectManager();
     return manager ? reinterpret_cast<Fn>(0x00603e90)(manager, nullptr) : nullptr;
 }
 

src/mods/autoattack.cpp

@@ -42,8 +42,10 @@ constexpr int kStartTicks = 5;
 // "Close range": where the hero touches a creature (game::ContactDistance,
 // heroes swing from 117..149) plus a step.
 constexpr int kStep = 100;
-// TODO(measure): the reach of ranged weapons (no ranged hero measured yet).
-constexpr int kRangedWeaponReach = 600;
+// Projectile weapons (game::HasRangedWeapon) have no reach: the hero fires
+// from where it stands. How far it looks for its next target with one is a
+// design value (same as the Blitzschlag chain / cast range).
+constexpr int kProjectileTargetRadius = 600;
 // An art order that has not started after this long is forgotten.
 constexpr DWORD kCastStartMs = 5000;
 
@@ -74,7 +76,7 @@ bool Idle(void* creature) {
 // How far the hero looks for its next target.
 double CloseRange(void* hero) {
     if (game::HasRangedWeapon(hero)) {
-        return kRangedWeaponReach;
+        return kProjectileTargetRadius;
     }
     return game::ContactDistance(hero, hero) + kStep;
 }

src/mods/inventory_save.cpp

@@ -25,6 +25,8 @@
 #include <nlohmann/json.hpp>
 
 #include "game/engine/events.h"
+#include "game/engine/items.h"
+#include "mods/potionstack.h"
 #include "mods/skillslots.h"
 #include "re/hook.h"
 #include "re/log.h"
@@ -242,7 +244,11 @@ json ToJson(const std::vector<uint8_t>& block, const uint8_t* shadow) {
     for (size_t i = 0; i < kGridCells; ++i) {
         const uint8_t* cell = cells + i * kGridCellSize;
         if (!AllZero(cell, kGridCellSize)) {
-            grid.push_back({{"cell", i}, {"item", U32At(cell)}, {"data", Hex(cell + 4, kGridCellSize - 4)}});
+            json entry = {{"cell", i}, {"item", U32At(cell)}, {"data", Hex(cell + 4, kGridCellSize - 4)}};
+            if (const unsigned count = mods::potionstack::StackCount(U32At(cell)); count > 1) {
+                entry["count"] = count;  // a potion stack (src/mods/potionstack.cpp)
+            }
+            grid.push_back(entry);
         }
     }
     j["grid"] = grid;
@@ -373,6 +379,9 @@ RE_HOOK(0x005def60, InvManager_Serialize);
 int __fastcall InvManager_Serialize(void* self, void* edx, void* stream, int write, int x) {
     ++g_heroFileDepth;
     g_readJson = g_readVanilla = 0;
+    if (!static_cast<char>(write)) {
+        mods::potionstack::ResetStacks();  // the ids of the previous game are gone
+    }
     const int result = InvManager_Serialize_original(self, edx, stream, write, x);
     --g_heroFileDepth;
     if (!static_cast<char>(write)) {
@@ -440,6 +449,12 @@ int __fastcall Inv_Serialize(void* self, void* edx, void* streamArg, int write)
         MemoryStream memory = MakeMemoryStream(block);
         Inv_Serialize_original(self, edx, &memory, 0);  // also clears the shadow (Clear)
         ++g_readJson;
+        for (const json& cell : j.at("grid")) {
+            if (cell.contains("count")) {
+                const size_t i = cell.at("cell");
+                mods::potionstack::SetStackCount(game::GridItem(static_cast<uint8_t*>(self), i), cell.at("count"));
+            }
+        }
         for (unsigned s = kGameSlots; s < kSlots; ++s) {
             const json& set = j.at("weaponSets").at(s);
             Inv_SetWeaponSetItem(self, edx, s, 0, set.at("hand0"), 0);

src/mods/potioncounts.cpp

@@ -0,0 +1,73 @@
+// Potion counts on the potion slots of the bottom bar.
+//
+// Request (2026-10-17): the potion selector always shows how many potions of
+// each kind the hero carries (only when there are any).
+//
+// The bar shows 5 potion kinds: cUI_Taskbar2::RenderPotionSlots (0x6e39e0)
+// draws control k-1 (embedded at +0x258, stride 0xbc) when
+// cInventoryEntry::GetPotionRef(k) finds a potion of kind k, else the empty
+// control behind it (+0x3ac). GetPotionRef walks the inventory grid and asks
+// ItemPotionKind for each item's kind; the count here does the same and adds
+// up the potion stacks (potionstack.cpp).
+
+#include <cstdint>
+#include <cwchar>
+
+#include "game/engine/items.h"
+#include "game/engine/objects.h"
+#include "game/ui/text.h"
+#include "mods/potionstack.h"
+#include "mods/skillslots.h"
+#include "re/hook.h"
+
+namespace mods::potioncounts {
+namespace {
+
+// cUI_Taskbar2 fields.
+constexpr size_t kInventoryIndex = 0x158;  // u16: the hero's inventory
+constexpr size_t kPotionSlots = 0x258;     // game::kPotionKinds embedded controls
+constexpr size_t kPotionSlotSize = 0xbc;
+
+constexpr uint16_t kCountFont = 1;  // as the art cooldown labels (skillbar.cpp)
+constexpr uint32_t kCountColor = 0xffffffff;
+constexpr int kCountInset = 3;  // from the slot's bottom-right corner
+
+void RenderCounts(uint8_t* taskbar, void* renderer) {
+    const uint8_t* inventory =
+        skillslots::Inventory(*reinterpret_cast<const uint16_t*>(taskbar + kInventoryIndex));
+    if (!inventory || !game::GetPlayerHero()) {
+        return;
+    }
+    unsigned counts[game::kPotionKinds + 1] = {};
+    game::ForEachGridItem(inventory, [&](uint32_t id) {
+        const int kind = game::PotionKindOf(id);
+        if (kind >= 1 && kind <= game::kPotionKinds) {
+            counts[kind] += potionstack::StackCount(id);
+        }
+    });
+    const int lineHeight = game::FontLineHeight(kCountFont);
+    for (int kind = 1; kind <= game::kPotionKinds; ++kind) {
+        if (counts[kind] == 0) {
+            continue;
+        }
+        game::UiRect rect = game::ControlScreenRect(taskbar + kPotionSlots + (kind - 1) * kPotionSlotSize);
+        rect.y += rect.h - lineHeight - kCountInset;
+        rect.w = static_cast<int16_t>(rect.w - kCountInset);
+        wchar_t text[8];
+        swprintf_s(text, L"%u", counts[kind]);
+        game::DrawText(renderer, kCountFont, rect, text, kCountColor, game::kTextRight);
+    }
+}
+
+} // namespace
+} // namespace mods::potioncounts
+
+// cUI_Taskbar2::RenderPotionSlots (0x6e39e0, ret 4): then the counts.
+void __fastcall UiTaskbar_RenderPotionSlots(void* self, void* edx, void* renderer);
+RE_HOOK(0x006e39e0, UiTaskbar_RenderPotionSlots);
+void __fastcall UiTaskbar_RenderPotionSlots(void* self, void* edx, void* renderer) {
+    UiTaskbar_RenderPotionSlots_original(self, edx, renderer);
+    if (self) {
+        mods::potioncounts::RenderCounts(static_cast<uint8_t*>(self), renderer);
+    }
+}

src/mods/potionstack.cpp

@@ -0,0 +1,321 @@
+// Potion stacks (request 2026-10-17, docs/potion-stacks.md).
+//
+// Potions of the same item type share one inventory cell, up to kMaxStack. A
+// stack is the one potion object in the cell plus a count kept here (by item
+// id, with the item type to tell a reused id apart) and saved with the JSON
+// inventories (src/mods/inventory_save.cpp).
+//
+// - Entry: a potion placed into the hero's grid (picked up, bought, dropped
+//   from the cursor; cInventoryEntry::PlaceItem 0x5d9ec0) pours into other
+//   stacks of its type that have room, on the next engine tick (the caller
+//   still uses the object while placing it). An emptied object is removed and
+//   destroyed.
+// - Sort button (cInventoryEntry::Sort 0x5db670, the inventory window's
+//   button): all potions of a type are poured together first.
+// - Drinking (cCreature::UseItem 0x54a8f0, measured): the potion's event
+//   handler (cPotion3D::vf18) applies the effect and marks it used up; UseItem
+//   then removes it from the grid (RemoveItem 0x5da5f0) and destroys it
+//   (cObjectManager::DestroyObject 0x5fbe10), and cEngine::UseActionSlot
+//   removes it once more. For a stack of more than one, the first removal
+//   takes one potion off the count instead, and the rest of that drink leaves
+//   the object alone.
+// - Load: cInventoryEntry::ReplaceItemId (0x5d80e0, hooked in skillslots.cpp;
+//   hero files map saved ids to the new objects) moves the count to the new id.
+// - Drawing: the count on each stack's cell, after cInventoryEntry::DrawItems
+//   (0x5dc3f0; cell rects from CellRect 0x5dd530).
+
+#include "mods/potionstack.h"
+
+#include <windows.h>
+
+#include <cwchar>
+#include <mutex>
+#include <unordered_map>
+#include <vector>
+
+#include "game/engine/events.h"
+#include "game/engine/items.h"
+#include "game/engine/objects.h"
+#include "game/engine/orders.h"
+#include "game/ui/text.h"
+#include "mods/skillslots.h"
+#include "re/hook.h"
+
+namespace mods::potionstack {
+namespace {
+
+constexpr unsigned kMaxStack = 20;
+
+constexpr uint16_t kCountFont = 1;  // as the potion slot counts (potioncounts.cpp)
+constexpr uint32_t kCountColor = 0xffffffff;
+constexpr int kCountInset = 2;
+
+// How long a drink's later removals of its stack are ignored.
+constexpr DWORD kDrinkMs = 100;
+
+struct Stack {
+    unsigned count;
+    uint32_t type;  // 0: not known yet (set on load before the objects exist)
+};
+
+std::mutex g_lock;
+std::unordered_map<uint32_t, Stack> g_stacks;
+
+// Potions placed into the hero's grid, poured on the next tick.
+std::vector<uint32_t> g_entered;
+// Inside cInventoryEntry::Sort (its re-placements are no entries).
+thread_local bool g_sorting = false;
+// The stack a drink just took a potion from (per thread: drinks come from the
+// engine and the UI thread).
+thread_local uint32_t g_drunk = 0;
+thread_local DWORD g_drunkAt = 0;
+
+uint32_t TypeOf(uint32_t id) {
+    const void* item = id ? game::GetObjectById(id) : nullptr;
+    return item ? game::ItemType(item) : 0;
+}
+
+uint8_t* HeroInventory() {
+    void* hero = game::GetPlayerHero();
+    return hero ? skillslots::Inventory(static_cast<uint32_t>(game::CreatureField(hero, game::kCreatureId)))
+                : nullptr;
+}
+
+bool IsHeroInventory(const void* inventory) {
+    return inventory && inventory == HeroInventory();
+}
+
+bool InGrid(const uint8_t* inventory, uint32_t id) {
+    bool found = false;
+    game::ForEachGridItem(inventory, [&](uint32_t item) { found = found || item == id; });
+    return found;
+}
+
+// Moves potions of `id` into the other stacks of its type in `inventory`
+// that have room (in grid order); an emptied `id` is removed and destroyed.
+void Pour(uint8_t* inventory, uint32_t id) {
+    const uint32_t type = TypeOf(id);
+    if (!type || !game::ItemPotionKind(type)) {
+        return;
+    }
+    const unsigned had = StackCount(id);
+    unsigned left = had;
+    game::ForEachGridItem(inventory, [&](uint32_t other) {
+        if (left == 0 || other == id || TypeOf(other) != type) {
+            return;
+        }
+        const unsigned count = StackCount(other);
+        const unsigned room = count < kMaxStack ? kMaxStack - count : 0;
+        const unsigned moved = left < room ? left : room;
+        if (moved) {
+            SetStackCount(other, count + moved);
+            left -= moved;
+        }
+    });
+    if (left == had) {
+        return;
+    }
+    if (left > 0) {
+        SetStackCount(id, left);
+        return;
+    }
+    using RemoveItemFn = int(__fastcall*)(void*, void*, uint32_t, uint32_t);
+    reinterpret_cast<RemoveItemFn>(0x005da5f0)(inventory, nullptr, id, 1);  // cInventoryEntry::RemoveItem
+    game::DestroyObject(id);
+}
+
+// Pours the potions from the back of the grid into those before them: full
+// stacks first.
+void StackAll(uint8_t* inventory) {
+    std::vector<uint32_t> potions;
+    game::ForEachGridItem(inventory, [&](uint32_t id) {
+        if (game::PotionKindOf(id)) {
+            potions.push_back(id);
+        }
+    });
+    for (size_t i = potions.size(); i-- > 0;) {
+        Pour(inventory, potions[i]);
+    }
+}
+
+void OnTick(uint8_t*) {
+    std::vector<uint32_t> entered;
+    {
+        std::lock_guard guard(g_lock);
+        entered.swap(g_entered);
+    }
+    uint8_t* inventory = entered.empty() ? nullptr : HeroInventory();
+    if (!inventory) {
+        return;
+    }
+    for (const uint32_t id : entered) {
+        if (InGrid(inventory, id)) {
+            Pour(inventory, id);
+        }
+    }
+}
+
+struct Registration {
+    Registration() { game::OnEngineTick(&OnTick); }
+} g_registration;
+
+void DrawCounts(void* inventory, void* renderer) {
+    auto* grid = static_cast<const uint8_t*>(inventory);
+    const int lineHeight = game::FontLineHeight(kCountFont);
+    const unsigned cells = game::GridCells(grid);
+    for (unsigned cell = 0; cell < cells; ++cell) {
+        const unsigned count = StackCount(game::GridItem(grid, cell));
+        if (count <= 1) {
+            continue;
+        }
+        using CellRectFn = game::UiRect*(__fastcall*)(void*, void*, game::UiRect*, uint32_t);
+        game::UiRect rect = {};
+        reinterpret_cast<CellRectFn>(0x005dd530)(inventory, nullptr, &rect, cell);  // cInventoryEntry::CellRect
+        rect.y += rect.h - lineHeight - kCountInset;
+        rect.w = static_cast<int16_t>(rect.w - kCountInset);
+        wchar_t text[8];
+        swprintf_s(text, L"%u", count);
+        game::DrawText(renderer, kCountFont, rect, text, kCountColor, game::kTextRight);
+    }
+}
+
+bool DrinkInProgress(uint32_t id) {
+    return id && id == g_drunk && GetTickCount() - g_drunkAt < kDrinkMs;
+}
+
+void Forget(uint32_t id) {
+    std::lock_guard guard(g_lock);
+    g_stacks.erase(id);
+}
+
+} // namespace
+
+unsigned StackCount(uint32_t id) {
+    if (!id) {
+        return 1;
+    }
+    Stack stack;
+    {
+        std::lock_guard guard(g_lock);
+        const auto it = g_stacks.find(id);
+        if (it == g_stacks.end()) {
+            return 1;
+        }
+        stack = it->second;
+    }
+    const uint32_t type = TypeOf(id);
+    if (stack.type == 0 && type != 0) {
+        std::lock_guard guard(g_lock);
+        g_stacks[id].type = type;  // first seen after loading
+        return stack.count;
+    }
+    return type == stack.type ? stack.count : 1;
+}
+
+void SetStackCount(uint32_t id, unsigned count) {
+    if (count <= 1) {
+        Forget(id);
+        return;
+    }
+    const uint32_t type = TypeOf(id);
+    std::lock_guard guard(g_lock);
+    g_stacks[id] = {count, type};
+}
+
+void OnItemIdReplaced(uint32_t oldId, uint32_t newId) {
+    std::lock_guard guard(g_lock);
+    const auto it = g_stacks.find(oldId);
+    if (it != g_stacks.end() && oldId != newId) {
+        const unsigned count = it->second.count;
+        g_stacks.erase(it);
+        g_stacks[newId] = {count, 0};  // type taken when first seen
+    }
+}
+
+void ResetStacks() {
+    std::lock_guard guard(g_lock);
+    g_stacks.clear();
+    g_entered.clear();
+}
+
+} // namespace mods::potionstack
+
+namespace game {
+
+using namespace mods::potionstack;
+
+// cInventoryEntry::PlaceItem (0x5d9ec0, ret 0x14): (cell, id, w, h, notify).
+int __fastcall Inv_PlaceItem(void* self, void* edx, uint32_t cell, uint32_t id, uint32_t w, uint32_t h, uint32_t notify);
+RE_HOOK(0x005d9ec0, Inv_PlaceItem);
+int __fastcall Inv_PlaceItem(void* self, void* edx, uint32_t cell, uint32_t id, uint32_t w, uint32_t h, uint32_t notify) {
+    const int previous = Inv_PlaceItem_original(self, edx, cell, id, w, h, notify);
+    if (!g_sorting && PotionKindOf(id) && IsHeroInventory(self)) {
+        std::lock_guard guard(g_lock);
+        g_entered.push_back(id);
+    }
+    return previous;
+}
+
+// cInventoryEntry::RemoveItem (0x5da5f0, ret 8): (id, notify).
+int __fastcall Inv_RemoveItem(void* self, void* edx, uint32_t id, uint32_t notify);
+RE_HOOK(0x005da5f0, Inv_RemoveItem);
+int __fastcall Inv_RemoveItem(void* self, void* edx, uint32_t id, uint32_t notify) {
+    if (DrinkInProgress(id)) {
+        return 1;
+    }
+    auto* item = static_cast<uint8_t*>(id ? GetObjectById(id) : nullptr);
+    auto* flags = item ? reinterpret_cast<uint32_t*>(item + kObjectFlags) : nullptr;
+    const unsigned count = flags && (*flags & kObjectUsedUp) ? StackCount(id) : 1;
+    if (count > 1) {
+        // A drink from a stack: one potion less, the object stays.
+        *flags &= ~kObjectUsedUp;
+        SetStackCount(id, count - 1);
+        g_drunk = id;
+        g_drunkAt = GetTickCount();
+        return 1;
+    }
+    return Inv_RemoveItem_original(self, edx, id, notify);
+}
+
+// cObjectManager::DestroyObject (0x5fbe10, ret 0x10): (id, 1, 0, 0) from UseItem.
+uint32_t __fastcall ObjMgr_DestroyObject(void* self, void* edx, uint32_t id, uint32_t a, uint32_t b, uint32_t c);
+RE_HOOK(0x005fbe10, ObjMgr_DestroyObject);
+uint32_t __fastcall ObjMgr_DestroyObject(void* self, void* edx, uint32_t id, uint32_t a, uint32_t b, uint32_t c) {
+    if (DrinkInProgress(id)) {
+        return 0;
+    }
+    Forget(id);
+    return ObjMgr_DestroyObject_original(self, edx, id, a, b, c);
+}
+
+// cCreature::UseItem (0x54a8f0, ret 4): a new drink starts.
+char __fastcall Creature_UseItem(void* self, void* edx, uint32_t id);
+RE_HOOK(0x0054a8f0, Creature_UseItem);
+char __fastcall Creature_UseItem(void* self, void* edx, uint32_t id) {
+    g_drunk = 0;
+    return Creature_UseItem_original(self, edx, id);
+}
+
+// cInventoryEntry::Sort (0x5db670, ret 4): stack the hero's potions first.
+void __fastcall Inv_Sort(void* self, void* edx, uint32_t mode);
+RE_HOOK(0x005db670, Inv_Sort);
+void __fastcall Inv_Sort(void* self, void* edx, uint32_t mode) {
+    if (IsHeroInventory(self)) {
+        StackAll(static_cast<uint8_t*>(self));
+    }
+    g_sorting = true;
+    Inv_Sort_original(self, edx, mode);
+    g_sorting = false;
+}
+
+// cInventoryEntry::DrawItems (0x5dc3f0, ret 8): (renderer, flag); then the counts.
+void __fastcall Inv_DrawItems(void* self, void* edx, void* renderer, uint32_t flag);
+RE_HOOK(0x005dc3f0, Inv_DrawItems);
+void __fastcall Inv_DrawItems(void* self, void* edx, void* renderer, uint32_t flag) {
+    Inv_DrawItems_original(self, edx, renderer, flag);
+    if (self) {
+        DrawCounts(self, renderer);
+    }
+}
+
+} // namespace game

src/mods/potionstack.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <cstdint>
+
+namespace mods::potionstack {
+
+// Potions of one type in the stack held by item `id` (1 for any other item).
+unsigned StackCount(uint32_t id);
+
+// Sets the stack size of item `id` (1 or less: no stack). Used when loading.
+void SetStackCount(uint32_t id, unsigned count);
+
+// cInventoryEntry::ReplaceItemId: the stack follows its item to the new id.
+void OnItemIdReplaced(uint32_t oldId, uint32_t newId);
+
+// Forgets all stacks (a save game is about to be loaded).
+void ResetStacks();
+
+} // namespace mods::potionstack

src/mods/skillslots.cpp

@@ -13,6 +13,7 @@
 // Network events sent from the shadow would name slot - 5: multiplayer with
 // more than 5 slots is not supported.
 
+#include "mods/potionstack.h"
 #include "mods/skillslots.h"
 
 #include <cstring>
@@ -202,6 +203,7 @@ void __fastcall Inv_ReplaceItemId(void* self, void* edx, int oldId, int newId);
 RE_HOOK(0x005d80e0, Inv_ReplaceItemId);
 void __fastcall Inv_ReplaceItemId(void* self, void*, int oldId, int newId) {
     WithShadow(Inv_ReplaceItemId_original, self, oldId, newId);
+    mods::potionstack::OnItemIdReplaced(static_cast<uint32_t>(oldId), static_cast<uint32_t>(newId));
 }
 
 void __fastcall Inv_FixInventory(void* self, void* edx);

tools/names.txt

@@ -102,6 +102,24 @@
 0x005dd9f0 cInventoryEntry::Serialize          # (stream, write)
 0x005de8a0 cInventoryEntry::FixInventory
 0x005da880 cInventoryEntry::GetPotionRef
+0x0043f630 ItemPotionKind                  # (ecx = item type table *0xaa9564, type): potion kind 1-5, 0 = none
+0x006e39e0 cUI_Taskbar2::RenderPotionSlots  # 5 potion kinds, controls at +0x258 (stride 0xbc)
+0x005d9ec0 cInventoryEntry::PlaceItem      # (cell, id, w, h, notify) -> network 0x13f; returns the item it replaced
+0x005da290 cInventoryEntry::GetItemAt      # (cell, resolve covered cells)
+0x005da360 cInventoryEntry::CountItems
+0x005da390 cInventoryEntry::RemoveAt       # (cell, notify) -> network 0x13e
+0x005da5f0 cInventoryEntry::RemoveItem     # (id, notify)
+0x005dab60 cInventoryEntry::FindFreeSpace  # (w, h, out cell, mode)
+0x005dac70 cInventoryEntry::FitsAt         # (cell, w, h)
+0x005db670 cInventoryEntry::Sort           # (mode): the inventory window's sort button, trader stock
+0x005dc3f0 cInventoryEntry::DrawItems      # (renderer, flag)
+0x005dd530 cInventoryEntry::CellRect       # (out rect, cell): 32x32 from the origin at +0x1164 / +0x1166
+0x0054a8f0 cCreature::UseItem              # (id): event 0x10 to the item; used up -> RemoveItem + DestroyObject
+0x005fbe10 cObjectManager::DestroyObject   # (id, 1, 0, 0)
+0x005ca9d0 cPotion3D::ReceiveEvent         # vf18: drink effect, sets +0x14 bit 0x8000000 (used up)
+0x00613040 cEngine::UseActionSlot          # (potion kind, ...): potion keys
+0x006c5f40 cUI_Inventory3::HandleEvent     # sort button (+0x158) -> cInventoryEntry::Sort
+0x006c1210 cUI_InventoryTab::Render        # grid origin (0, 0x188), DrawItems
 0x0057f750 SlotCountForLevel                   # table 0x95503c, max 5
 0x008110d0 GameWindowProc                    # in-game window procedure: keys (dead keys, modifiers 0x182cdec)
 0x005def60 cInventoryManager::Serialize      # (stream, write, x): all 32 inventories, save games