Another macOS behaviour required for DO, I appreciate expandable folders are new to the DO world and most users are still oblivious to it, but macOS Finder has had this for 40 years so there's a few things DO can learn from Finder.
In the Finder, when the user has a directory selected Option+Right arrow will expand the folder and all its sub folders.
Same with multiple folders selected, Option+Right Arrow expands all recursively.
To do the reverse, Option+Left Arrow to close all folders, recursively.
Super useful.