Don't preselect popup menu items on macOS

Native macOS context menus are shown slightly to the right of the mouse
cursor so that no item is initially selected. Other platforms do the
same thing.

Adjust placement position manually to ensure the same behavior when
popping up the menu manually.

src/osx/window_osx.cpp

@@ -866,9 +866,10 @@ bool wxWindowMac::SetCursor(const wxCursor& cursor)
 bool wxWindowMac::DoPopupMenu(wxMenu *menu, int x, int y)
 {
 #ifndef __WXUNIVERSAL__
+    const wxPoint mouse = wxGetMousePosition();
+
     if ( x == wxDefaultCoord && y == wxDefaultCoord )
     {
-        wxPoint mouse = wxGetMousePosition();
         x = mouse.x;
         y = mouse.y;
     }
@@ -876,6 +877,14 @@ bool wxWindowMac::DoPopupMenu(wxMenu *menu, int x, int y)
     {
         ClientToScreen( &x , &y ) ;
     }
+
+    if ( x == mouse.x )
+    {
+        // move the menu just off the cursor so that no menu item is pre-selected,
+        // for consistency with native popups and other platforms
+        x += 1;
+    }
+
     menu->GetPeer()->PopUp(this, x, y);
     return true;
 #else