Running a room on one clock
Put this page on the projector or the shared screen, then send the link to everyone who wants the countdown on their own laptop. The first person into an empty room gets the controls; everyone else watches. If the facilitator changes partway through a workshop, the new one presses Take control and the old panel turns back into a display. There is only ever one controller, so two people cannot fight over the clock.
The room never sends ticks. It broadcasts the instant the countdown ends, and each screen counts down to that instant against its own clock. That keeps the traffic to a handful of small messages per session, and it means a screen that wakes from sleep or reconnects after a dropout shows the right number immediately instead of resuming a stale count.
When time runs out the clock keeps going and turns amber, counting up as overtime. That is usually more useful than a timer that stops at zero: the room can see how far past the slot it has gone. The message banner and the flash toggle are there for the moment before that, when you want the group to start landing the discussion without interrupting whoever is speaking.