The TV shows the deck; the phone is the clicker

You're presenting at a client's office. Their conference-room TV has a browser and nothing else. You have a phone and a deck.

The deck is open in LibreOffice Impress on a display in your container. On the TV, you open the display URL with ?readonly=true&sharing=true. On your phone, the same URL with ?sharing=true. The TV shows the slides. Your phone shows the same slides, and it accepts touch. Swipe right: the TV advances. Both are viewing the same desktop session.

A colleague dialing in from another city opens the same URL. They see the slide change the moment you swipe.

No HDMI dongle, no AirPlay, no "can everyone see my screen." The presentation runs in neither room. Both screens are just viewers.

The presentation runs in neither room. Both screens are just viewers.