When you open a changeset in Visual Studio 2013 the details are displayed in the Team Explorer window. In the Related Work Items area are displayed all work items associated with that changeset.
On mouse-over that work item you can see the details (title, state, assigned to, ...) of the work item in a small window.
It's possible to customize that window?
I'd like to display some custom fields that we have in the task work item.
There's no officially supported way to do that (no public API). If you are willing to accept the risk of your extension breaking in a future update, you can certainly hook in and modify it.