Patch the persistent Test Send window outside of PI System Explorer
From tech support case 00602329:
In PI System Explorer, click on an element that has notification rules set up. Go to the Notification Rules tab, select a notification rule, click on Manage Formats, and click on Test Send. Enter an email address and click on Test Send. While the button still says "Testing", switch to another window. When the send is complete, the Test Send window will appear overtop of the window that you switched to. The only way to make it go away is to switch back to the PI System Explorer window.
This situation came up when I tried to send myself a test email. Naturally, after clicking on Test Send, I immediately switched to Microsoft Outlook and waited for the email. Then the Test Send window appeared and I was confused.
Please fix this issue.