The menu is disabled because a modal dialog box doesn't allow the focus to move beyond the confines of the dialogs frame which includes menus. This is modal as defined by MS in MSDN.

A window or form is modal if it retains the focus until you explicitly close
it. Dialog boxes and messages are usually modal. For example, in a modal dialog
box, you can't access another window until you choose OK in the dialog box.

To stop this behaviour you need to set your forms WindowType property to 0 (zero) indicating a modeless state.