win32: fix combo% on-popup

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

@@ -454,16 +454,21 @@
      (define/public (append-combo-item s)
        (SendMessageW/str combo-hwnd CB_ADDSTRING 0 s))
      (define/public (clear-combo-items)
-       (void))
+       (SendMessageW combo-hwnd CB_RESETCONTENT 0 0))
 
      (define/public (on-popup) (void))
 
      (define/override (is-command? cmd)
-       (= cmd CBN_SELENDOK))
+       (or (= cmd CBN_SELENDOK)
+           (= cmd CBN_DROPDOWN)))
 
      (define/public (do-command cmd control-hwnd)
-       (let ([i (SendMessageW combo-hwnd CB_GETCURSEL 0 0)])
-         (queue-window-event this (lambda () (on-combo-select i)))))
+       (cond
+        [(= cmd CBN_SELENDOK)
+         (let ([i (SendMessageW combo-hwnd CB_GETCURSEL 0 0)])
+           (queue-window-event this (lambda () (on-combo-select i))))]
+        [(= cmd CBN_DROPDOWN)
+         (constrained-reply (get-eventspace) (lambda () (on-popup)) (void))]))
 
      (define/override (is-hwnd? a-hwnd)
        (or (ptr-equal? panel-hwnd a-hwnd)

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

@@ -15,10 +15,6 @@
 (provide 
  (protect-out choice%))
 
-(define CBN_DROPDOWN        7)
-(define CBN_CLOSEUP         8)
-(define CBN_SELENDCANCEL    10)
-
 (define choice% 
   (class item%
     (init parent cb label

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

@@ -616,6 +616,11 @@
 (define CB_INSERTSTRING #x014A)
 (define CB_SETCURSEL #x014E)
 (define CB_GETCURSEL #x0147)
-(define CBN_SELENDOK 9)
 (define CB_ADDSTRING #x0143)
 (define CB_RESETCONTENT #x014B)
+
+(define CBN_SELENDOK 9)
+(define CBN_DROPDOWN        7)
+(define CBN_CLOSEUP         8)
+(define CBN_SELENDCANCEL    10)
+