Scrolledwindow

from gi.repository import Gtk as gtk
from gi.repository import Gdk as gdk

class app:
    def __init__(self):
        window_main = gtk.Window()
        window_main.set_size_request(gdk.Screen.get_default().get_width(),0)
        window_main.set_name("window_main")
        window_main.move(0,0)
        window_main.set_keep_above(True)
        window_main.set_resizable(False)
        window_main.set_decorated(False)
        window_main.set_border_width(0)
        window_main.connect("destroy",lambda e: gtk.main_quit())

        #create widgets for "categories place"#
        box_main = gtk.HBox()

        scrolledwindow_categories = gtk.ScrolledWindow()
        scrolledwindow_categories.set_policy(
            gtk.PolicyType.AUTOMATIC,
            gtk.PolicyType.NEVER
            )
        scrolledwindow_categories.set_size_request(20,20)

        viewport_categories = gtk.Viewport()
        viewport_categories.add(box_main)
        scrolledwindow_categories.add(viewport_categories)
        window_main.add(scrolledwindow_categories)

        notebook_categories = gtk.Notebook()
        box_main.add(notebook_categories)

        for i in range(20):
            button = gtk.Button("Button")
            notebook_categories.append_page(button,gtk.Label("Text"))

        window_main.show_all()
        gtk.main()

app()