Just now, while editing a couple of notes, I used Ctrl-Z to recover a recent delete and was surprised that it took action on the main source code I was working on.
Thinking it was a focus issue I tried a few time clicking on the scribble text before actually doing the Ctrl-Z, same result though - the Ctrl-Z acted on the main editor. Then I tried with Ctrl-Y to redo, and that too acted on the main editor. Normal cut/copy/paste actions are local to scribble though.
I submit that, if implementing the Undo and Redo functions in the scribble box is too complicated, at least disabling those functions in the main editor would be wise. After all, the scribble box could be covering the code editor's window.
Geany: 1.38 GTK+ v3.24.34 and GLib v2.74.3 Slackware64 15.0