metric: Time to Interactive about lighthouse HOT 4 CLOSED

1. Main thread is ~available
   • Calculation TBD.

Maybe this is when there's enough yielding that the UI can change or interact or show progress every 16.7ms (i.e. 60 FPS)?

5. Any other ideas?

Maybe mention that deferred work may be scheduled and run while not affecting TTI (as long as it's not clogging things up)?

/cc @natduca @esprehn

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I've updated the definition of the TTI above metric based on latest discussions with @benshayden, @skyostil, japhet@, @pbakaus and others.

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Are we sure TTI should block on domContentLoadedEventEnd? I find that many pages start to become interactable before parsing is complete. If for instance there's a script in the footer inserted via doc.write, this can delay DCL firing when the CPU will be idle and the page interactive.

That said, I think it's also reasonable to discourage pages from running the parser a long time after their content is usable - we can message to devs that they shouldn't do this and if they do so, our loading indicators may stay visible longer than they would like. But that also means that some existing content will continue to show a loading indicator / not be considered interactive until after it has reached the interactive state.

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Are we sure TTI should block on domContentLoadedEventEnd?

Hah. Pat pretty much the same comment yesterday on the Progressive page Load Metrics doc yesterday(https://docs.google.com/document/d/1oiy0_ych1v2ADhyG_QW7Ps4BNER2ShlJjx2zCbVzVyY/edit#)

Quoting him, then me..

This isn't necessarily tied to anything visual. A site can have a lot of js at the end of the body that needs to get loaded/parsed/executed that will delay DCL but may not cause any visual change to the page, though by-definition if you are waiting for the progress indicator this may be included anyway as part of that logic.

Fair. This isn't directly a visual dependency, so I'll adjust that.

I agree that DCL be a direct dependency here as its not visual. Main thread availability will tackle CPU contention, and layout stability for the rest of the blocking script impact.

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