show as many items as possible on macOS 10.13+

on these systems scrollbars on comboboxes are not always shown, even if the system is configured to do so, in these cases show all possible to avoid the impression of a limited subset

see https://github.com/wxWidgets/wxWidgets/pull/1534/files

src/osx/cocoa/combobox.mm

@@ -20,6 +20,7 @@
     #include "wx/dcclient.h"
 #endif
 
+#include "wx/osx/private/available.h"
 #include "wx/osx/cocoa/private/textimpl.h"
 
 // work in progress
@@ -347,7 +348,10 @@ wxWidgetImplType* wxWidgetImpl::CreateComboBox( wxComboBox* wxpeer,
 {
     NSRect r = wxOSXGetFrameForControl( wxpeer, pos , size ) ;
     wxNSComboBox* v = [[wxNSComboBox alloc] initWithFrame:r];
-    [v setNumberOfVisibleItems:13];
+    if (WX_IS_MACOS_AVAILABLE(10, 13))
+        [v setNumberOfVisibleItems:999];
+    else
+        [v setNumberOfVisibleItems:13];
     if (style & wxCB_READONLY)
         [v setEditable:NO];
     wxNSComboBoxControl* c = new wxNSComboBoxControl( wxpeer, v );