When I click on the block, it doesn't report anything (undefined).
When I put the block in another block (e.g. say), I get a list, which is the desired result.
How does that happen?
I think I have figured it out: if a reporter is an input to another block and it reports undefined, it gets reevaluated. Clicking on an isolated reporter that reports undefined only evaluates it once and doesn't show a result speech bubble thingy. I knew those facts previously, but how does it not work when I click on it more than once?
It changes the state of the current process object. Each time I click on it (isolated), a new process object is created, and since it's only evaluated once, the function only works with the process object's initial state. If the reporter is in another script, it works with the process object's initial state (reports undefined), so then it's reevaluated with the same process object's changed state (reports a list of coordinates).