I have a Grid containing some Labels inside Frames to make it look like a table. This grid is inserted in a vertical Box in which direct child Labels are centered correctly (they are packed in the box in the same way as the Grid).
My simplified code is this:
import gi
gi.require_version("Gtk", "3.0")
from gi.repository import Gtk
window = Gtk.Window()
g = Gtk.Grid() # this should be in the horizontal center of the window
g.attach(Gtk.Label("This should be centered but it is not."), 0, 0, 1, 1)
b = Gtk.Box(orientation=Gtk.Orientation.VERTICAL)
b.pack_start(g, True, False, 0) # The same behavior with: b.pack_start(g, True, True, 0)
b.pack_start(Gtk.Label("This label is centered as it should be. Try to resize the window."), True, False, 0)
window.add(b)
window.connect("delete-event", Gtk.main_quit)
window.show_all()
Gtk.main()
and this is the GUI it produces:
