As fleshed out as a mind map can get, sometimes you just want to associate a hidden note to a node, like to house an explanation of what the node is.

It’s not conceptually hard; just add an Excel-like popup note to a node. It can be as simple as modifying your sticky notes assets as a new class that shows and hides on click or hover. DON’T cover the screen with  fixed-sized icons. Just add a scaling dot or some notation that there is an associated note hiding in the node. See Excel notes as the example.