For each item on the CopyPatrol interface, there is a pane that can be opened up with an open/closed toggle.

For each link that has a potential match, there will be a Compare toggle button, and this text structure: "xx% (xx words) URL" -- with the percent of the match, the number of matching words, and the source URL.

When there's more than one potential source, the Compare buttons and links stack; see wireframes below. Sources are listed from the highest match to the lowest.

When the user clicks on the Compare button, the item expands to show a pane showing two columns -- the article and the potential source. The display is similar to Earwig's comparison tool, but with a scroll bar for each column, so the user can line up the matched text side-by-side.

The pane can be closed by clicking the Compare button again.

If a pane is open and the user clicks on a different Compare button, the open pane closes, and the new one opens. There should only be one open pane at a time.

If there's a noticeable delay in loading the comparison, show a spinner to indicate that it's working.

Wireframes:

To get the comparison data from Earwig's API, simply pass detail=true when using action=compare. For example:
https://tools.wmflabs.org/copyvios/api.json?version=1&action=compare&detail=true&project=wikipedia&lang=en&title=User:EarwigBot/Copyvios/Tests/2&url=https://www.whitehouse.gov/administration/president-obama&format=jsonfm
For more info on the API, see https://tools.wmflabs.org/copyvios/api.