The jenkins maintenance yesterday evening implemented an authorization configuration change to allow authenticated users the ability to cancel a running build (DM-9713).
To cancel a build from the Blue Ocean UI either click the “stop” icon on the right hand side of the job overview (which lists all builds) or the same icon on the top right-ish of a build:
Is it possible to add a confirmation dialog for stopping jobs. It’s easy to accidentally press that button and kill someone’s job that has been running happily for hours 
Well, anything is possible. The blueocean front end is writen in react.js, which I am not familiar with, but I imagine confirmation could be implemented completely client side.
The classic UI does use a confirmation dialog. I’ve opened a feature request upstream but if similar functionality hasn’t already been implemented in BO, it seems likely it is a conscious design choice.
