BSERV-10278, webhooks are looking great and we're excited to integrate pull request support.
There doesn't seem to be a pull request event for when the pull request is updated. For example, somebody changes the title. Bitbucket Cloud has this as a pull request updated event:
GitHub's is pull request edited:
There also doesn't seem to be an event for when the proposed head changes, like if somebody pushes extra commits onto the pull request branch. GitHub calls a "synchronize" event, and includes the pull request details including the new head commit sha for merging, another subtype of their pull request event. I'm not sure if Bitbucket Cloud sends this event. However it's pretty critical for rebuilding pull requests if the proposed code changes.