Thanks to Harold for his wishlist and thanks to Enrico for implementing...
I have a wishlist item too. jEdit has a plugin called TaskList which inspects each file periodically (actually only at save time) to look for special comments: eg in C or Java types // TODO: something // NOTE: something // BUG: fix this
The keywords are configurable by the user. (I suppose the single line comment start character(s) could be as well; depending on language.)
Anyway, the lines with these comments show up in the TaskList tab in the lower window. In Geany's case it would be a new tab along with Scribble, Compiler, etc.
In that window, then, clicking on one of the entries would take one to that line in the source code, like a compiler error message or the "Find Usage" function that Geany has.
I had been wanting to learn enough about GTK programming to do it myself, but haven't had the time yet. So I throw the idea out there...