phlib: Implement horizontal scrolling via Shift + Mousewheel

phlib/extlv.c

@@ -813,6 +813,28 @@ LRESULT CALLBACK PhpExtendedListViewWndProc(
             context->WindowDpi = listviewDpi;
         }
         break;
+    case WM_MOUSEWHEEL:
+        {
+            if (LOWORD(wParam) & MK_SHIFT)
+            {
+                SHORT wheelDelta = GET_WHEEL_DELTA_WPARAM(wParam);
+                ULONG wheelScrollChars;
+
+                if (!PhGetSystemParametersInfo(SPI_GETWHEELSCROLLCHARS, 0, &wheelScrollChars, 0))
+                    wheelScrollChars = 3;
+
+                LONG charsToScroll = (LONG)wheelScrollChars * -wheelDelta / WHEEL_DELTA;
+                LONG count = charsToScroll < 0 ? -charsToScroll : charsToScroll;
+
+                for (LONG i = 0; i < count; i++)
+                {
+                    SendMessage(WindowHandle, WM_HSCROLL, charsToScroll > 0 ? SB_LINERIGHT : SB_LINELEFT, 0);
+                }
+
+                return 0;
+            }
+        }
+        break;
     }
 
     return CallWindowProc(oldWndProc, WindowHandle, WindowMessage, wParam, lParam);

phlib/theme.c

@@ -5629,7 +5629,9 @@ LRESULT CALLBACK PhpThemeWindowACLUISubclassProc(
     switch (uMsg)
     {
     case WM_VSCROLL:
+    case WM_HSCROLL:
     case WM_MOUSEWHEEL:
+    case WM_MOUSEHWHEEL:
         InvalidateRect(WindowHandle, NULL, FALSE);
         break;
     case WM_NOTIFY:

phlib/treenew.c

@@ -2023,8 +2023,12 @@ VOID PhTnpOnMouseWheel(
     )
 {
     // The normal mouse wheel can affect both the vertical scrollbar and the horizontal scrollbar,
-    // but the vertical scrollbar takes precedence.
-    if (Context->VScrollVisible)
+    // but the vertical scrollbar takes precedence unless the Shift key is held for horizontal scrolling.
+    if ((VirtualKeys & MK_SHIFT) && Context->HScrollVisible)
+    {
+        PhTnpProcessMouseHWheel(Context, -Distance);
+    }
+    else if (Context->VScrollVisible)
     {
         PhTnpProcessMouseVWheel(Context, -Distance);
     }