Comment 7 for bug 669537

After more investigation it turns out that the code that actually emits the focus out event to the spinbutton was introduced with revision 8855 in file src/widgets/toolbox.cpp

prev version:
static void connector_tb_event_attr_changed(Inkscape::XML::Node *repr,
                                            gchar const *name, gchar const * /*old_value*/, gchar const * /*new_value*/,
                                            bool /*is_interactive*/, gpointer data)
{
    GtkWidget *tbl = GTK_WIDGET(data);

    if (g_object_get_data(G_OBJECT(tbl), "freeze")) {
        return;
    }
    if (strcmp(name, "inkscape:connector-spacing") != 0) {
        return;
    }

    GtkAdjustment *adj = (GtkAdjustment*)
            gtk_object_get_data(GTK_OBJECT(tbl), "spacing");
    gdouble spacing = defaultConnSpacing;
    sp_repr_get_double(repr, "inkscape:connector-spacing", &spacing);

    gtk_adjustment_set_value(adj, spacing);
    gtk_adjustment_value_changed(adj);

    spinbutton_defocus(GTK_OBJECT(tbl));
}

new version:

static void connector_tb_event_attr_changed(Inkscape::XML::Node *repr,
                                            gchar const *name, gchar const * /*old_value*/, gchar const * /*new_value*/,
                                            bool /*is_interactive*/, gpointer data)
{
    GtkWidget *tbl = GTK_WIDGET(data);

    if (g_object_get_data(G_OBJECT(tbl), "freeze")) {
        return;
    }
    if (strcmp(name, "inkscape:connector-spacing") == 0)
    {
        GtkAdjustment *adj = (GtkAdjustment*)
                gtk_object_get_data(GTK_OBJECT(tbl), "spacing");
        gdouble spacing = defaultConnSpacing;
        sp_repr_get_double(repr, "inkscape:connector-spacing", &spacing);

        gtk_adjustment_set_value(adj, spacing);
        gtk_adjustment_value_changed(adj);
    }

    spinbutton_defocus(GTK_OBJECT(tbl));
}

but I cannot figure out why this change was made. If there is no reason for this maybe we could revert it?