A possible option is to intercept and re-route mouse messages to your 'sizing' control, eg. (test):
Code:
MSG Msg;
while (GetMessage(&Msg,NULL,0,0)>0)
{
switch (Msg.message)
{
case WM_MOUSEMOVE:
case WM_LBUTTONDOWN:
case WM_LBUTTONUP:
case WM_RBUTTONDOWN:
case WM_RBUTTONUP:
if (Msg.hwnd==hAttached)
{
Msg.hwnd=hSizingCntrl;
}
}
TranslateMessage(&Msg);
DispatchMessage(&Msg);
}
where hAttached is the control your 'sizing' control is attached to and hSizingCntrl is the 'sizing' control itself. A WH_GETMESSAGE hook might be a better way to go with this approach (see also Hooks).