Google Tag Manager recently updated the preview or the debug mode. It is also called Tag Assistant, not to be confused with the Tag Assitant Chrome extension.
Major Differences between old and new versions
- The previous version of Google Tag Manager preview mode opened up a small window at the bottom of your website, while this one opens the debugger in a new window or tab.
- The old one replied on 3rd party cookie while the new one does not
- The old one was difficult to use when the links were clicked as the preview window refreshed but the new one is in a new window/tab so it shows you the entire history, even when a new page load up.
Check out this video for details