No it won't, the other application advertising new clipboard contents won't offer the "SCINTILLARECTANGLE" target (unless its another Scintilla which copied in rectangle mode), so Scintilla won't paste as a rectangle.
OK, I didn't know this "target" thing.
PS "target" is a GTK name for "data format", why they can't just call it that I don't know, maybe an X11 thing from the mists of time.
Is it documented somewhere how to define and use these "targets" and how this whole thing works?