I recently started using the r2019b release and noticed that an additional hidden handle is included in GUIs created in GUIDE if the GUI contains axes that are children of a UI panel. The handle is "handles.scribeOverlay" which is an annotationPane object (matlab.graphics.shape.internal.AnnotationPane).
Tested in both r2019a and r2019b, I've noticed that the handle appears when....
Opened in: r2019a r2019b
Created in: r2019a No Yes
Created in: r2019b Yes Yes
To demonstrate (if you're interested), attached are two simple, nearly identical GUIDE-GUIs (figures & m-files), one of which was created in r2019a and the other in r2019b (see filenames). They mereley contain a UI panel and an embedded axis. The opening functions print a list of all handles in the 'handles' structure, even the hidden object handles, to the command window. If these GUIs are run in r2019b, both will show that the scribeOverlay is present but if run in 2019a, only the "r2019b" version will show this handle.
My questions are, why is there an annotationPane suddenly appearing and what purpose does it serve? Turning its visibility on/off doesn't reveal any differences to the GUI.