When you first open a Replay, you'll be able to toggle between the Viewer and the DevTools. The Viewer is where you're able to watch a full video replay.

You can play or scrub the video and click anywhere on the Replay to add a comment. It's important to note that comments are associated with both a position on the Replay and a timestamp. In the image below, you can see that comments are added to the scrubber at the bottom, and clicking on a comment in the sidebar or the comment icon in the scrubber will bring you back to that place and time.

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Comments added from the DevTools panel will also show up on the Viewer panel. You can see an example Replay here with a comment identifying an issue from the viewer and a follow up identifying the bug from the code.