Widgets extending comboitem such as bandbox, combobox, etc will display a popup.
The popup will be displayed:

• below the input if space is available
• if not, above the popup if space is available,
• if not, below the popup, overflowing if necessary.

The slide-in animation for the popup opening action is always: slide down from top to bottom.
This is fine when the popup is below the anchor, but could be improved when the popup is above the anchor by sliding the popup upward, from bottom to top.