Is this a bug, or a feature? Redirect on contribution landing page triggering while editing
Looking at the demo site, you are inserting JavaScript directly into the thank-you message:

&lt;p&gt;Thanks for supporting our organization with your membership. You can learn more about membership benefits from our members only page.&lt;/p&gt;

&lt;p&gt;&lt;br /&gt;
&lt;script type="text/javascript"&gt;
&lt;!--
window.location = "https://civicrm.org/"
//--&gt;
&lt;/script&gt;&lt;/p&gt;


CKEditor renders the whole thing when the tab is loaded or the field edited. This causes the JS to run, which fires the redirect. It's technically a feature that CKEditor allows us to enter JavaScript and executes it for us, but I'd never recommend doing this. It will become a huge pain to troubleshoot later, when someone other than you goes to edit the thank-you email receipt and it keeps kicking them to a survey page. I recommend moving your redirect into a template override or an extension, so that it only loads when the user gets to the actual thank-you page.