3 files changed, 18 insertions, 24 deletions
diff --git a/win32/PlatWin.cxx b/win32/PlatWin.cxx
index 87840ff3b..bb808b4e7 100644
--- a/win32/PlatWin.cxx
+++ b/win32/PlatWin.cxx
@@ -2895,7 +2895,7 @@ class ListBoxX : public ListBox {
 	PRectangle rcPreSize;
 	Point dragOffset;
 	Point location;	// Caret location at which the list is opened
-	int wheelDelta; // mouse wheel residue
+	MouseWheelDelta wheelDelta;
 	ListOptions options;
 	DWORD frameStyle = WS_THICKFRAME;
 
@@ -2927,7 +2927,7 @@ public:
 		desiredVisibleRows(9), maxItemCharacters(0), aveCharWidth(8),
 		parent(nullptr), ctrlID(0), dpi(USER_DEFAULT_SCREEN_DPI),
 		delegate(nullptr),
-		widestItem(nullptr), maxCharWidth(1), resizeHit(0), wheelDelta(0) {
+		widestItem(nullptr), maxCharWidth(1), resizeHit(0) {
 	}
 	ListBoxX(const ListBoxX &) = delete;
 	ListBoxX(ListBoxX &&) = delete;
@@ -3689,21 +3689,15 @@ LRESULT ListBoxX::WndProc(HWND hWnd, UINT iMessage, WPARAM wParam, LPARAM lParam
 		}
 		return ::DefWindowProc(hWnd, iMessage, wParam, lParam);
 	case WM_MOUSEWHEEL:
-		wheelDelta -= GET_WHEEL_DELTA_WPARAM(wParam);
-		if (std::abs(wheelDelta) >= WHEEL_DELTA) {
+		if (wheelDelta.Accumulate(wParam)) {
 			const int nRows = GetVisibleRows();
 			int linesToScroll = std::clamp(nRows - 1, 1, 3);
-			linesToScroll *= (wheelDelta / WHEEL_DELTA);
+			linesToScroll *= wheelDelta.Actions();
 			int top = ListBox_GetTopIndex(lb) + linesToScroll;
 			if (top < 0) {
 				top = 0;
 			}
 			ListBox_SetTopIndex(lb, top);
-			// update wheel delta residue
-			if (wheelDelta >= 0)
-				wheelDelta = wheelDelta % WHEEL_DELTA;
-			else
-				wheelDelta = - (-wheelDelta % WHEEL_DELTA);
 		}
 		break;
 
diff --git a/win32/PlatWin.h b/win32/PlatWin.h
index 893618b38..68098f9c7 100644
--- a/win32/PlatWin.h
+++ b/win32/PlatWin.h
@@ -49,6 +49,20 @@ int SystemMetricsForDpi(int nIndex, UINT dpi) noexcept;
 
 HCURSOR LoadReverseArrowCursor(UINT dpi) noexcept;
 
+class MouseWheelDelta {
+	int wheelDelta = 0;
+public:
+	bool Accumulate(WPARAM wParam) noexcept {
+		wheelDelta -= GET_WHEEL_DELTA_WPARAM(wParam);
+		return std::abs(wheelDelta) >= WHEEL_DELTA;
+	}
+	int Actions() noexcept {
+		const int actions = wheelDelta / WHEEL_DELTA;
+		wheelDelta = wheelDelta % WHEEL_DELTA;
+		return actions;
+	}
+};
+
 #if defined(USE_D2D)
 extern bool LoadD2D();
 extern ID2D1Factory *pD2DFactory;
diff --git a/win32/ScintillaWin.cxx b/win32/ScintillaWin.cxx
index 9ffd40d06..c405ed8d1 100644
--- a/win32/ScintillaWin.cxx
+++ b/win32/ScintillaWin.cxx
@@ -276,20 +276,6 @@ public:
 	}
 };
 
-class MouseWheelDelta {
-	int wheelDelta = 0;
-public:
-	bool Accumulate(WPARAM wParam) noexcept {
-		wheelDelta -= GET_WHEEL_DELTA_WPARAM(wParam);
-		return std::abs(wheelDelta) >= WHEEL_DELTA;
-	}
-	int Actions() noexcept {
-		const int actions = wheelDelta / WHEEL_DELTA;
-		wheelDelta = wheelDelta % WHEEL_DELTA;
-		return actions;
-	}
-};
-
 struct HorizontalScrollRange {
 	int pageWidth;
 	int documentWidth;