HUD: inventory drag & drop, UI hit tests, centred overlays

- Mouse move events carry previous and current x (+4, +0xc); button events
  x at +8. All are shifted, and restored to the saved values.
- The cursor object's previous x (+0xc) is shifted too: picking up an item
  writes the cursor position into the event (0x6559a0).
- GetMouseClientPos returns the shifted position inside a moved window.
- Window hit tests (vf1c 0x731d40, taskbar 0x6dfb10) called outside a window
  scope, e.g. 'is UI under the cursor' for world picking (0x75a370), use the
  offset the top-level window was last drawn with.
- The item carried by the cursor (0x6555e0) and the UI manager's centred
  status line / message box (0x75bbf0, 0x75c200) draw with the centre offset.

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

src/mods/hud.cpp

@@ -54,37 +54,49 @@ int OffsetFor(Anchor anchor) {
     return anchor == Anchor::Center ? (width - kLayoutWidth) / 2 : width - kLayoutWidth;
 }
 
-// Whether the current thread has already shifted the mouse cursor position.
-thread_local bool g_cursorShifted = false;
+// Offset by which the current thread has shifted the mouse cursor (0: none).
+thread_local int g_cursorDx = 0;
 
 // RAII: shift the mouse cursor object's x by -dx for the scope (outermost
 // window only). Windows poll it for hover (highlights, tooltips) while they
-// render and handle events. The window thread may store a new position
-// meanwhile; that one is kept instead of restoring.
+// render and handle events: +4 is the current x, +0xc the one before the last
+// update (read by 0x6559a0, e.g. when an inventory item is picked up). The
+// window thread may store a new position meanwhile; that one is kept instead
+// of restoring.
 class CursorOffset {
 public:
     explicit CursorOffset(int dx) {
         auto* cursor = *reinterpret_cast<uint8_t**>(game::addr::g_mouseCursor);
-        if (dx == 0 || g_cursorShifted || !cursor) {
+        if (dx == 0 || g_cursorDx != 0 || !cursor) {
             return;
         }
-        x_ = reinterpret_cast<volatile int32_t*>(cursor + 4);
-        original_ = *x_;
-        shifted_ = original_ - dx;
-        *x_ = shifted_;
-        g_cursorShifted = true;
+        fields_[0].Shift(cursor + 0x4, dx);
+        fields_[1].Shift(cursor + 0xc, dx);
+        g_cursorDx = dx;
     }
     ~CursorOffset() {
-        if (x_) {
-            InterlockedCompareExchange(reinterpret_cast<volatile LONG*>(x_), original_, shifted_);
-            g_cursorShifted = false;
+        if (g_cursorDx != 0 && fields_[0].x) {
+            fields_[0].Restore();
+            fields_[1].Restore();
+            g_cursorDx = 0;
         }
     }
 
 private:
-    volatile int32_t* x_ = nullptr;
-    int32_t original_ = 0;
-    int32_t shifted_ = 0;
+    struct Field {
+        volatile LONG* x = nullptr;
+        LONG original = 0;
+        LONG shifted = 0;
+
+        void Shift(uint8_t* address, int dx) {
+            x = reinterpret_cast<volatile LONG*>(address);
+            original = *x;
+            shifted = original - dx;
+            *x = shifted;
+        }
+        void Restore() { InterlockedCompareExchange(x, original, shifted); }
+    };
+    Field fields_[2];
 };
 
 // Whether the current thread is inside a hooked window's render method.
@@ -121,12 +133,51 @@ void TrackHover(void* window, int dx) {
     }
 }
 
+// Offsets top-level windows were last drawn with, for calls made outside their
+// scope (hit tests by the UI manager). Most recent first.
+struct ControlOffset {
+    const void* control;
+    int dx;
+};
+constexpr int kControlOffsetCount = 64;
+ControlOffset g_controlOffsets[kControlOffsetCount] = {};
+SRWLOCK g_controlOffsetsLock = SRWLOCK_INIT;
+
+void RecordOffset(const void* control, int dx) {
+    AcquireSRWLockExclusive(&g_controlOffsetsLock);
+    int i = 0;
+    while (i < kControlOffsetCount - 1 && g_controlOffsets[i].control != control) {
+        ++i;
+    }
+    for (; i > 0; --i) {
+        g_controlOffsets[i] = g_controlOffsets[i - 1];
+    }
+    g_controlOffsets[0] = {control, dx};
+    ReleaseSRWLockExclusive(&g_controlOffsetsLock);
+}
+
+int OffsetOf(const void* control) {
+    AcquireSRWLockShared(&g_controlOffsetsLock);
+    int dx = 0;
+    for (const ControlOffset& entry : g_controlOffsets) {
+        if (entry.control == control) {
+            dx = entry.dx;
+            break;
+        }
+    }
+    ReleaseSRWLockShared(&g_controlOffsetsLock);
+    return dx;
+}
+
 // RAII: shift drawing by dx for the scope. Windows nest (children, shared base
 // methods), so only the outermost window decides.
 class DrawOffset {
 public:
     DrawOffset(void* window, int dx) : outermost_(!g_drawScoped), cursor_(outermost_ ? dx : 0) {
         if (outermost_) {
+            if (window) {
+                RecordOffset(window, dx);
+            }
             TrackHover(window, dx);
             g_drawScoped = true;
             g_offsetX = dx;
@@ -144,42 +195,57 @@ private:
     CursorOffset cursor_;
 };
 
-// Mouse events (cEvent_mouseMove / _mouseButton_down / _mouseButton_up): vf04
-// returns 1 / 2 / 3, the cursor position in render pixels is at +8 / +0xc.
-// Returns the x field, or nullptr for other events (keys, UI commands, ...).
-int32_t* MouseEventX(void* event) {
+// Mouse events, by vf04 category (positions in render pixels):
+//   1 cEvent_mouseMove:          previous x, y at +4 / +8, current at +0xc / +0x10
+//   2 cEvent_mouseButton_down,
+//   3 cEvent_mouseButton_up:     x, y at +8 / +0xc
+// Stores the offsets of the x fields; returns their count (0: no mouse event).
+int MouseEventXOffsets(void* event, int (&offsets)[2]) {
     using CategoryFn = int(__fastcall*)(void* self, void* edx);
     auto category = reinterpret_cast<CategoryFn>((*static_cast<void***>(event))[1]);
-    const int value = category(event, nullptr);
-    if (value < 1 || value > 3) {
-        return nullptr;
+    switch (category(event, nullptr)) {
+    case 1:
+        offsets[0] = 0x4;
+        offsets[1] = 0xc;
+        return 2;
+    case 2:
+    case 3:
+        offsets[0] = 0x8;
+        return 1;
+    default:
+        return 0;
     }
-    return reinterpret_cast<int32_t*>(static_cast<uint8_t*>(event) + 8);
 }
 
 // RAII: shift the mouse position by -dx for the scope (outermost window only).
 class EventOffset {
 public:
     EventOffset(void* event, int dx) : cursor_(dx) {
-        if (dx != 0 && !g_eventShifted) {
-            x_ = MouseEventX(event);
-            if (x_) {
-                dx_ = dx;
-                *x_ -= dx_;
-                g_eventShifted = true;
-            }
+        if (dx == 0 || g_eventShifted) {
+            return;
         }
+        int offsets[2];
+        count_ = MouseEventXOffsets(event, offsets);
+        for (int i = 0; i < count_; ++i) {
+            x_[i] = reinterpret_cast<int32_t*>(static_cast<uint8_t*>(event) + offsets[i]);
+            original_[i] = *x_[i];
+            *x_[i] = original_[i] - dx;
+        }
+        g_eventShifted = count_ > 0;
     }
     ~EventOffset() {
-        if (x_) {
-            *x_ += dx_;
+        if (count_ > 0) {
+            for (int i = 0; i < count_; ++i) {
+                *x_[i] = original_[i];
+            }
             g_eventShifted = false;
         }
     }
 
 private:
-    int32_t* x_ = nullptr;
-    int dx_ = 0;
+    int count_ = 0;
+    int32_t* x_[2] = {};
+    int32_t original_[2] = {};
     CursorOffset cursor_;
 };
 
@@ -245,6 +311,10 @@ HRESULT __stdcall DrawIndexedPrimitiveShifted(void* device, DWORD type, DWORD fv
 
 } // namespace
 
+int CursorOffsetX() {
+    return g_cursorDx;
+}
+
 void OnDevice(void* device) {
     static void* hooked = nullptr;
     if (!device || device == hooked) {
@@ -375,3 +445,67 @@ int __fastcall UiTooltip_Render(void* self, void* edx, void* device) {
 }
 
 } // namespace game
+
+namespace mods::hud {
+
+// Window hit test (vf1c: is the point inside, positions relative to the parent
+// at +0x50). The UI manager asks it with plain render coordinates, e.g. whether
+// the world under the cursor is covered by UI (0x75a370); translate those by
+// the offset the top-level window was drawn with. Calls from inside a moved
+// window's scope already use its layout.
+int HitTestX(void* window, int x) {
+    if (g_drawScoped || g_eventShifted || g_cursorDx != 0) {
+        return x;
+    }
+    auto* root = static_cast<uint8_t*>(window);
+    while (auto* parent = *reinterpret_cast<uint8_t**>(root + 0x50)) {
+        root = parent;
+    }
+    return x - OffsetOf(root);
+}
+
+} // namespace mods::hud
+
+namespace game {
+
+int __fastcall UiWindow_HitTest(void* self, void* edx, int x, int y);
+RE_HOOK(0x00731d40, UiWindow_HitTest);
+int __fastcall UiWindow_HitTest(void* self, void* edx, int x, int y) {
+    return UiWindow_HitTest_original(self, edx, mods::hud::HitTestX(self, x), y);
+}
+
+int __fastcall UiTaskbar_HitTest(void* self, void* edx, int x, int y);
+RE_HOOK(0x006dfb10, UiTaskbar_HitTest);
+int __fastcall UiTaskbar_HitTest(void* self, void* edx, int x, int y) {
+    return UiTaskbar_HitTest_original(self, edx, mods::hud::HitTestX(self, x), y);
+}
+
+// The item carried by the mouse cursor (0x6555e0, drawn by the render loop
+// after the UI) is placed like UI items: relative to x = 512 of the layout.
+// Items are only picked up from centred windows.
+int __fastcall MouseCursor_DrawItem(void* self, void* edx, int a, int b);
+RE_HOOK(0x006555e0, MouseCursor_DrawItem);
+int __fastcall MouseCursor_DrawItem(void* self, void* edx, int a, int b) {
+    mods::hud::DrawOffset offset(nullptr, mods::hud::OffsetFor(Anchor::Center));
+    return MouseCursor_DrawItem_original(self, edx, a, b);
+}
+
+// UI manager overlays laid out around x = 512 (0x75bbf0 status/progress line,
+// 0x75c200 message box above the taskbar).
+int __fastcall UiManager_DrawStatusLine(void* self, void* edx, int a, int b);
+RE_HOOK(0x0075bbf0, UiManager_DrawStatusLine);
+int __fastcall UiManager_DrawStatusLine(void* self, void* edx, int a, int b) {
+    mods::hud::DrawOffset offset(nullptr, mods::hud::OffsetFor(Anchor::Center));
+    return UiManager_DrawStatusLine_original(self, edx, a, b);
+}
+
+int __fastcall UiManager_DrawMessage(void* self, void* edx, int a, int b, int c);
+RE_HOOK(0x0075c200, UiManager_DrawMessage);
+int __fastcall UiManager_DrawMessage(void* self, void* edx, int a, int b, int c) {
+    mods::hud::DrawOffset offset(nullptr, mods::hud::OffsetFor(Anchor::Center));
+    return UiManager_DrawMessage_original(self, edx, a, b, c);
+}
+
+} // namespace game
+
+