rhythmbox should allow multiple discontinuous genre/artist/album selections

Thanks for the bug report. It works charmingly nice for me. Could you elaborate
on what you are exactly trying to do or maybe provide a screenshot?

works fine for me too

This is what happens:

Artist list:
*All
Bar
Baz
Foo

click Foo

Artist list:
All
Bar
Baz
*Foo

hold down control and click Bar

Artist list:
All
Bar
Baz
*Foo

the last artist list should be:
All
*Bar
Baz
*Foo

It seems to be working now.

It stopped working again and I found (what I think is) the problem by accident.

When mouse highlight on control is enabled (System -> Preferences -> Mouse ->
Cursors -> the checkbox at the bottom), it doesn't work (or it works at a few
"random" times) and when it's disabled it works consistently.

I think this might be a problem with X? something? not passing the control-click
to rhythmbox, but I'm not sure, so I'll leave this bug under rhythmbox because
it's the only place I've noticed this problem.

By the way, the relevent code is:

widgets/rb-property-view.c:
rb_property_view_set_selection_mode (RBPropertyView *view,
                                     GtkSelectionMode mode)
{
        g_return_if_fail (mode == GTK_SELECTION_SINGLE || mode ==
GTK_SELECTION_MULTIPLE);
        gtk_tree_selection_set_mode (gtk_tree_view_get_selection (GTK_TREE_VIEW
(view->priv->treeview)),
                                     mode);
}

and

sources/rb-library-source.c:
rb_property_view_set_selection_mode (source->priv->XXXXXX, GTK_SELECTION_MULTIPLE);

where XXXXXX is one of genre, album, and artist.

Thanks David for investigating, however I'm not quite sure, what you intend us
to do now?

I want control-clicking to work when the mouse highlight is enabled. I don't
know whether the problem is in rhythmbox, gtk, or the highlighter program, but
if it's in the highlighter program, I think it should do something like:

grabKey(<control>)
grabKey(<control-release>)
if (key == <control>)
  passToApplication(<control>)
  previousKey = <control>
if (key == <control-release>)
  passToApplication(<control-release>)
  if (previousKey == <control>)
    highlightMouse()

In essence, if the problem is in the <control>-grabber program because it
doesn't pass all keys to the application or waits for a control-release to pass
the key(s), it should pass all keys unconditionally.

The package that does the highlighting is gnome-control-center. The grab code is
in gnome-settings-daemon/gnome-settings-mouse.c:

if (keyval == GDK_Control_L || keyval == GDK_Control_R)
        {
          if (xev->type == KeyPress)
            {
              XAllowEvents (gdk_x11_get_default_xdisplay (),
                            SyncKeyboard,
                            xev->xkey.time);
            }
          else
            {
              XAllowEvents (gdk_x11_get_default_xdisplay (),
                            AsyncKeyboard,
                            xev->xkey.time);
              gnome_settings_locate_pointer (screen);
            }
        }
      else
        {
          XAllowEvents (gdk_x11_get_default_xdisplay (),
                        ReplayKeyboard,
                        xev->xkey.time);
          XUngrabKeyboard (gdk_x11_get_default_xdisplay (),
                           xev->xkey.time);
        }

I found documentation of XAllowEvents with the {Replay,Sync,Async}Keyboard
options at http://tronche.com/gui/x/xlib/input/XAllowEvents.html and it looks
like the problem is with SyncKeyboard ("At this time, the keyboard again appears
to freeze."). I really don't know much about how X works, so I might be way off
mark.

Right, that's a duplicate of bug #9441 which is known upstream:
http://bugzilla.gnome.org/show_bug.cgi?id=125618. Feel free to comment on the
upstream bug

This bug has been marked as a duplicate of bug 9441.