win32: combo-field% on-popup

original commit: 7571579ce5a9a0b20c83a5a64ff0e985030ce121

collects/mred/private/wx/win32/canvas.rkt

@@ -47,6 +47,8 @@
 (define HTHSCROLL           6)
 (define HTVSCROLL           7)
 
+(define CB_SHOWDROPDOWN #x014F)
+
 (define-cstruct _SCROLLINFO
   ([cbSize _UINT]
    [fMask _UINT]
@@ -488,6 +490,9 @@
         [(= cmd CBN_DROPDOWN)
          (constrained-reply (get-eventspace) (lambda () (on-popup)) (void))]))
 
+     (define/public (popup-combo)
+       (SendMessageW combo-hwnd CB_SHOWDROPDOWN 1 0))
+
      (define/override (is-hwnd? a-hwnd)
        (or (ptr-equal? panel-hwnd a-hwnd)
            (ptr-equal? canvas-hwnd a-hwnd)