I have Telegram installed as a flatpak (AFAIK there’s no other way), and if I configure it to autostart by making a symlink of /var/lib/flatpak/exports/share/applications/org.telegram.desktop.desktop to ~/.config/autostart, it does start with GNOME, but Telegram’s window gets “stuck”: even though it is fully functional, I can’t move it around by dragging its title bar. If I restart it after gnome session is up and running, all works as expected.
Nice tip, thanks! One thing that did seem to make a difference was enabling Settings > Advanced > System Integration > Use system window frame which forces it to use GNOME’s windows decorations. Now I can interact with Telegram’s window as expected. Still, there seems to exist some sort of delay for it to fully respond: for the first couple of seconds, clicking on the topbar icon (which still relies on Topicons Fix extension BTW) does nothing. So I guess I’ll combine both solutions (GNOME’s decorations + autostart delay)