Opener

Opener[x]
represents an opener with setting x, displayed as

when x is True and

when x is False.

Opener[Dynamic[x]]
takes the setting to be the dynamically updated current value of x, with the value of x being toggled if the opener is clicked.

Dynamically change a variable:

Out[1]=

Change the appearance:

By default the opener changes only when you click it:

By setting AutoAction, the opener toggles as the mouse moves over the opener area:

Change the background color:

Align with surrounding text:

By default Opener is enabled:

By setting Enabled->False, the opener is disabled but visible in its current state:

Include margins around the Opener:

Use Opener with PaneSelector for hiding and exposing additional controls:

Opener is a special case of Toggler: