A mred:dialog-box% is a dialog box which can correctly handle container objects (on some platforms, dialogs are resizable, and these dialogs prevent the user from sizing them too small). It must contain a single panel, which must be a descendant of mred:panel%.
Passes all arguments on to wx:dialog-box%'s super-init, then calls the dialog's centre method with wx:const-both, centering the dialog in the screen (or its parent, if non-null).