Fix position of popup menus in wxGenericListCtrl

The position was off by the header height due to a missing translation
between wxGenericListCtrl and wxListMainWindow, in which the menu is
actually shown, coordinates.

Closes #18133.

src/generic/listctrl.cpp

@@ -5467,6 +5467,12 @@ wxColour wxGenericListCtrl::GetForegroundColour() const
 bool wxGenericListCtrl::DoPopupMenu( wxMenu *menu, int x, int y )
 {
 #if wxUSE_MENUS
+    // Coordinates here are given in this control coordinates system, but
+    // PopupMenu() wants them in the coordinates system of the window it's
+    // called on, so transform them.
+    ClientToScreen(&x, &y);
+    m_mainWin->ScreenToClient(&x, &y);
+
     return m_mainWin->PopupMenu( menu, x, y );
 #else
     return false;