(This is cross-posted from https://mathematica.stackexchange.com/questions/283142/how-get-this-kind-of-group-opener.)

Some notebooks of mine have Section (and Subsection) cells formatted like the following, so that clicking on the icon to the right of the Section's title opens the entire group of cells in that Section:

But I cannot reproduce this behavior in a new notebook. What is the magic setting in the Option Inspector to get this kind of icon to open groups?

With the Default stylesheet, the desired icons, inline to the right of the Section titles, automatically appear when the Section groups are closed.

However, I am already using a custom stylesheet here — a variant of David's Presentations stylesheet. So the question amounts to asking what else I need to do to get that displayed group opener.

Related: https://mathematica.stackexchange.com/questions/272106/how-restore-default-stylesheets-new-type-of-group-openers.

Unfortunately, the method I showed there in my own answer to the question no longer seems to work.