Fix synthetic mouse events produced by wxAutoScrollTimer

Simplify the code translating from screen to client coordinates and make
it correct by using ScreenToClient() directly instead of emulating it
manually, which doesn't work quite right, at least in wxGTK.

Closes #22289.

src/generic/scrlwing.cpp

@@ -147,12 +147,7 @@ void wxAutoScrollTimer::Notify()
 
             // the mouse event coordinates should be client, not screen as
             // returned by wxGetMousePosition
-            wxWindow *parentTop = m_win;
-            while ( parentTop->GetParent() )
-                parentTop = parentTop->GetParent();
-            wxPoint ptOrig = parentTop->GetPosition();
-            event2.m_x -= ptOrig.x;
-            event2.m_y -= ptOrig.y;
+            m_win->ScreenToClient(&event2.m_x, &event2.m_y);
 
             event2.SetEventObject(m_win);