Solved.

In a topic elsewhere, I wrongly assumed using the space bar was the correct way to accept what's highlighted in the screenshot below:

Actual result: the highlighted context menu item is ignored, not accepted.

I think of this as a UX bug. Maybe from NodeBB mimicing the UX of Discourse … I vaguely recall having this problem years ago. Too late to resolve, attempting to the change the default behaviour would cause uproar.