Pasted

#include <gtk/gtk.h>
#include <stdbool.h>
 
bool on_button_clicked(GtkWidget *button, gpointer data);
static bool on_cell_renderer_text_editing_started(
        GtkCellRenderer *renderer, GtkCellEditable *editable, 
        gchar *path, gpointer data);
 
int main(int argc, char *argv[]) {
    gtk_init(&argc, &argv);
    
    // Creating entry
    GtkWidget *entry = gtk_entry_new();
    GtkWidget *open_button = gtk_button_new_with_mnemonic("_Open");
    
    g_signal_connect(G_OBJECT(open_button), "clicked",
             G_CALLBACK(on_button_clicked), entry);
    
    // Creating tree view stuff
    GtkListStore *store = gtk_list_store_new(2, G_TYPE_INT, G_TYPE_STRING);
    GtkWidget *tree_view = gtk_tree_view_new();
    gtk_tree_view_set_model(GTK_TREE_VIEW(tree_view), GTK_TREE_MODEL(store));
    // Creating columns
    GtkCellRenderer *renderer = gtk_cell_renderer_text_new();
    gtk_tree_view_insert_column_with_attributes(GTK_TREE_VIEW(tree_view),
            -1, "Number", gtk_cell_renderer_text_new(),
            "text", 0, NULL);
    gtk_tree_view_insert_column_with_attributes(GTK_TREE_VIEW(tree_view),
            -1, "Right", renderer,
            "text", 1,
            NULL);
    g_object_set(G_OBJECT(renderer), "editable", true, NULL);
    // Populating models
    GtkTreeIter iter;
    gtk_list_store_append(store, &iter);
    gtk_list_store_set(store, &iter, 0, 42, 1, "right 1", -1);
    gtk_list_store_append(store, &iter);
    gtk_list_store_set(store, &iter, 0, 13, 1, "right 2", -1);
    gtk_list_store_append(store, &iter);
    gtk_list_store_set(store, &iter, 0, 9, 1, "right 3", -1);
    // Callback for editing text
    g_signal_connect(
            G_OBJECT(renderer), "editing-started", 
            G_CALLBACK(on_cell_renderer_text_editing_started), NULL);
    
    // Packing together
    GtkWidget *window = gtk_window_new(GTK_WINDOW_TOPLEVEL),
            *hbox = gtk_hbox_new(false, 0),
            *vbox = gtk_vbox_new(false, 0);
    gtk_container_add(GTK_CONTAINER(hbox), entry);
    gtk_container_add(GTK_CONTAINER(hbox), open_button);
    gtk_container_add(GTK_CONTAINER(vbox), hbox);
    gtk_container_add(GTK_CONTAINER(vbox), tree_view);
    gtk_container_add(GTK_CONTAINER(window), vbox);
    
    gtk_widget_show_all(window);
    
    gtk_main();
    
    return 0;
}
 
bool on_button_clicked(GtkWidget *button, gpointer data) {
    GtkWidget *entry = data,
            *subwindow = gtk_window_new(GTK_WINDOW_TOPLEVEL),
            *sublabel = gtk_label_new("This is a rather long message\n"
                    "with many lines whose mere purpose is\n"
                    "to test some stuff. It will appear in \n"
                    "the RIGHT PLACE.");
    gtk_container_add(GTK_CONTAINER(subwindow), sublabel);
    gtk_window_set_decorated(GTK_WINDOW(subwindow), FALSE);
    gtk_window_set_modal(GTK_WINDOW(subwindow), TRUE);
 
 
    GtkWidget *window = gtk_widget_get_toplevel(entry);
    gtk_window_set_transient_for(
            GTK_WINDOW(subwindow), GTK_WINDOW(window));
    
    gint dx, dy;
    gtk_window_get_position(GTK_WINDOW(window), &dx, &dy);
    
    GtkAllocation allocation;
    gtk_widget_get_allocation(entry, &allocation);
    
    gtk_window_move(
            GTK_WINDOW(subwindow), 
            allocation.x+dx, allocation.y+allocation.height*2+dy);
    
    gtk_widget_show_all(subwindow);
    return false;
}
 
static bool on_cell_renderer_text_editing_started(
        GtkCellRenderer *renderer, GtkCellEditable *editable, 
        gchar *path, gpointer data) {
    GtkWidget *subwindow = gtk_window_new(GTK_WINDOW_TOPLEVEL),
            *sublabel = gtk_label_new("This is a rather long message\n"
                    "with many lines whose mere purpose is\n"
                    "to test some stuff. It will appear in\n"
                    "the WRONG PLACE!");
    gtk_container_add(GTK_CONTAINER(subwindow), sublabel);
    gtk_window_set_decorated(GTK_WINDOW(subwindow), FALSE);
    gtk_window_set_modal(GTK_WINDOW(subwindow), TRUE);
 
    GdkWindow *window = gtk_widget_get_window(GTK_WIDGET(editable));
    gtk_window_set_transient_for(
            GTK_WINDOW(subwindow), GTK_WINDOW(window));
 
    gint dx, dy;
    gtk_window_get_position(GTK_WINDOW(window), &dx, &dy);
 
    GtkAllocation allocation;
    gtk_widget_get_allocation(GTK_WIDGET(editable), &allocation);
    
    gtk_window_move(
            GTK_WINDOW(subwindow),
           allocation.x+dx, allocation.y+allocation.height*2+dy);
    gtk_widget_show_all(subwindow);
    return false;
}