Grouping issues by milestone in epic

Currently in an epic, you can re-order issues in the issues list. This gives flexibility for indicating priority or order of work.
In some other use cases, we want to view the issues separated by milestone. This will make it very easy (and automated) to which issues are upcoming and which have been done in future and previous milestones.
How can we solve this problem of being able to track issues in the epic by milestone? How can we still have re-ordering capability? What happens to milestones that have no start date or no end date or not both?

How should issues be viewed in an epic, so that you can see the milestone that they belong to.

https://gitlab.com/gitlab-org/gitlab-ee/issues/6086 already plans to show the milestone assigned to epic issues mockup

For a first iteration, let's show different headers for milestones inside the epic issues list:

Edited Sep 09, 2019 by Keanon O'Keefe