CQ used to do that automatically, but it caused problems to experimental tryjobs.  We completely killed that feature instead of improving logic. But manual user cancel is feasible. That said, it won't be in CQ, but in its integration with codereview. And hence, I mark dual-list it into Gerrit.

A good feature for the gerrit buildbucket plugin after the migration/launch is complete.

Removing Milestone-Afterglow, as it has ceased to have meaning. More refined milestones may be added back in the near future.