By default, your normal items within a mega submenu will be visible when the submenu is opened. By leaving the Submenu Type set to “Auto”, these column items become stacks. If you switch the Submenu Type to Mega or Flyout, you will get a dropdown instead, and this article deals with that situation.
Flyouts within Mega Submenus are not recommended
I do not generally recommend using dropdowns within mega submenus. This is generally confusing for users and leads to bad UX, so you should consider whether this is really the best format for your submenu.
Generally, better formats would include using Tabs or else Flyout submenus as your first level and mega submenus as your second level. These formats achieve similar effects (hidden content revealed on activation within a submenu) in a more structured format.
Enabling Dropdowns within Mega Submenus
If you wish to allow dropdowns within a mega submenu, you can enable that in the Control Panel > Main UberMenu Configuration > Submenus > Allow Dropdown within Mega Submenu
Please note: if you enable this setting, you cannot use the “Slide” submenu transition. This is because the slide transition requires hidden overflow to work, while dropdowns within mega submenus requires visible overflow to work. If you want to use this setting, you must switch to a Transition other than Slide
Multi-level mega submenus (intended use case)
One good use (really the intended one) for this is to allow a single-line submenu to have additional mega submenus so that you can have a double-level mega menu. This is a very specific scenario, however.
Here’s the result when using the setting as intended: