It is very common to want to CI build/test the merged result of a PR.

While Stash show's the build status of commits, and will show the build status for the HEAD commit of */pr/${pullRequestId}/from on the PR page, it would be great be able to show the build status for the HEAD commit of */pr/${pullRequestId}/merge on the PR page.

https://developer.atlassian.com/stash/docs/latest/how-tos/updating-build-status-for-commits.html#Viewing_build_results
https://developer.atlassian.com/static/rest/stash/3.11.3/stash-build-integration-rest.html

A REST API to PUT a build status for a CommitId /rest/build-status/1.0/commits/{commitId} exists, and this will work for the HEAD commit of */pr/${pullRequestId}/from. It might be nice if that also worked and displayed for HEAD commit of */pr/${pullRequestId}/merge.

Alternatively the Build Status of a PR could be considered as distinct from the Build Status of a Commit, and
/rest/api/1.0/projects/{projectKey}/repos/{repositorySlug}/pull-requests/{pullRequestId}
extended with a '/build-status' API to set this status.