This bug is likely to appear only on Mac (multiple window mode). By default, the "New Form" dialog appears at startup in Designer. If the "New Form" dialog is disabled and Designer is started without a file (i.e. by launching Designer itself rather than attempting to open a .ui file), the only top level menu shown is the Designer application menu (the one that says "Designer"). The File menu, Edit menu, etc., are not visible.

Switching to a different application (such as by Command-Tab) and then back to Designer results in the missing menus becoming visible.