I'd like to set the poster frame for some of my videos to make it easier to see what they are at a glance. As this answer describes, this used to be possible using the gear menu and then later using a pop-up on the timeline in edit mode.
In MacOS Monterey, neither of these options seem to work. The timeline editor still behaves the same way, but the popup to set the poster frame doesn't exist:
Sometimes if I drag the start time around and then exit the editor the poster frame changes, but it doesn't seem to happen reliably or predictably.
How can I set the poster frame for a video with Photos.app in MacOS Monterey?