Now I think I got it.
On the side bar there are four areas:
one "standard", outside of the higlighted part (no diff); clicking on it does nothing;
one higlighted which, when the mouse is on it, shows a pop-up with diff infos;
a third area around the arrow, highlighted, that, when the mouse is on it, switches to "not highlighted" and shows the same pop-up as above; clicking on it starts the copy;
a fourth area, very thin, between the arrow and the upper "standard" (non higlighted) part, that, when the mouse is on it, switches the arrow to "not highlighted" but does not show the pop-up and, when clicked, generates the "Encountered an improper argument" message box.
Call me picky!
