NEED HELP: Animated Dropdown Description

I’m trying to make an animated dropdown description, under the list item (inside the vertical box), that takes no space underneath until called.
It’s dropdown action is called from a function inside my CPP class.
Currenlt Tree:

|Vertical_Box
|_ListItem
|__ListItemStuff
|_DropdownDescription (Overlay)
|__WidgetSwitcher
|___TextOnly
|____TextBlock
|___WPictures
|____TextBlock
|____Picture

Basically I want it to have it extend to match the length of the description.

The trick with Visibility (Collapsed to Visible) makes the list item visually glitch for a while, I want it to look natural.