First of all, I think there are two type of project.One is open-source project, other is enterprise project.
I understand in an open-source project, reviewer may want to know if a PR has passed CI check, then he open that PR and review code. If CI check failed, he will not look that PR at all.
But in an enterprise project, PRs are all in plan. Reviewer will review PR anyway. CI check is a useful tool for reviewer confirm PR’s effect.
At now, from Travis CI doc:
Travis CI also currently doesn’t build pull requests when the upstream branch is updated, as this would lead to an excessive number of new builds.
I understand that. But I think it will be nice for having an option that will mark the PR all passed status to OUTDATE, when the upstream branch is updated.
And maybe, for an enterprise project, when there are more than one PR, we can manually start PR CI. Because their status will be OUTDATE when any of them merged into upstream.