I would like to be able to get at “screenshot” of the entire page, not only the visible part.
This feature would probably only be available for chrome and firefox and could trigger a shipped screenshot plugin that saves entire page and uploads it in addition to the normal screenshot.
There is an existing issue to track that: Ability to get fullpage screenshots · Issue #50 · WPO-Foundation/webpagetest · GitHub but “it’s complicated” would be an understatement, particularly when dealing with infinite scroll, lazy loaders, etc.
i use fireshot plugin. It works with IE, FF and Chrome. It’s pretty easy to use.