I asked about this on the gtk-list but got no responses responses that
would indicate why a 'STRING' target is required when dragging text between
two Gtk widgets in the same app.  I believe it may be a bug in Gtk 1.3.x
drag-n-drop, so file it here.

The following simple Python script demonstrates that a drag-and-drop
target of STRING is always needed when copying text between two widgets.
A 'text/plain' target will not work.  If you run this script and set fail
to 0 it will work.  If you set fail to 1 it will not.  The only difference
is that in the failure case only a 'text/plain' target is given.  In the
non-failure case, only a 'STRING' target is given.  It will also fail if
you define the targets list to have both 'STRING' and 'text/plain'
targets, but make the 'text/plain' target higher priority than the 
'STRING' target.

--
Skip Montanaro
skip@pobox.com


#!/usr/bin/env python

import sys
import gtk

fail = 0

def main():
    win = gtk.GtkWindow()
    win.connect('destroy', gtk.mainquit)
    table = gtk.GtkTable(2,2, gtk.TRUE)
    win.add(table)

    if fail:
        targets = [('text/plain', 0, 0)]
    else:
        targets = [('STRING', 0, 0)]

    b = gtk.GtkEntry()
    b.set_text("drag me ...")
    b.set_name("entry source")
    b.drag_source_set(gtk.GDK.BUTTON1_MASK,
                      targets,
                      (gtk.GDK.ACTION_DEFAULT|
                       gtk.GDK.ACTION_COPY))
    establish_notify(b)
    table.attach(b, 0, 1, 0, 1)
    
    b = gtk.GtkEntry()
    b.set_text('... here ...')
    b.set_name("entry dest")
    b.drag_dest_set(gtk.GTK.DEST_DEFAULT_ALL,
                    targets,
                    (gtk.GDK.ACTION_DEFAULT|
                     gtk.GDK.ACTION_COPY))
    establish_notify(b)
    table.attach(b, 1, 2, 0, 1)

    b = gtk.GtkButton(label='Quit')
    b.connect("clicked", gtk.mainquit)
    b.set_name("quit button")
    table.attach(b, 0, 2, 1, 2)

    win.show_all()
    gtk.mainloop()
    
def establish_notify(w):
    for signal in ["drag-begin","drag-data-delete","drag-data-get",
                   "drag-data-received","drag-drop","drag-end",
                   "drag-leave","drag-motion"]:
        w.connect(signal, note_signal, signal)

last_signal = ""
def note_signal(w, context, *args):
    global last_signal
    
    sig = (w.get_name(), args[-1])
    if sig == last_signal:
        sys.stdout.write(".")
        sys.stdout.flush()
    else:
        sys.stdout.write("\n%s" % (sig,))
        sys.stdout.flush()
        last_signal = sig
    if args[-1] == "drag-end":
        sys.stdout.write("\n")

    return gtk.FALSE

if __name__ == '__main__':
    main()