This is a workaround; under normal circumstances the discard changes dialog does not appear when leaving a page edit.
Here are the steps to discard all unpublished changes:
Edit the page and get to a resume draft mode
Edit the page with the Unpublished Changes lozenge
Refresh the page
Before the refresh the URL should look similar to:
After Refreshing, it should now appear similar to:
Trigger Close the Editor Dialog
Type a character on the page
The Close the Editor dialogue should now appear
Select Discard all changes to discard all changes made since last publish, Show changes to see the changes, or Keep draft.