AWS Cloudfront distribution staying "in Progress" for 55 minutes and counting

CloudFront is notoriously slow at reporting that changes to a distribution's configuration are fully deployed... but testing typically reveals that the new behavior is in place long before the state change occurs.

CloudFront doesn't pre-load your content from S3, so the time is completely unrelated to the total size of the objects -- all that is happening in the "In Progress" state is that the configuration (origins, cache behaviors, etc.) is replicating globally to all of the 100+ edge locations. An ususually long delay only implies that it hasn't fully finished propagating the configuration, for whatever internal reason.