I'm using Gtk.Fixed and a viewport putting images that, sometimes, overlaps. How can I set the z-index like in html?
Gtk.Fixed doesn't support setting the z-order. It's not meant for overlapping widgets, so the z-order is arbitrary and probably depends on what order the widgets were added in.
It's not clear from your question what your intention is; if you are purposely overlapping widgets, use Gtk.Overlay instead. If you are not intending to overlap them, then you should use a more versatile container such as Gtk.Grid which will take the widgets' sizes into account.
PyGObject: Gtk.Fixed Z-order