Developers expect the button to take a screenshot of the entire page. If there's overflow and scrollbars are present, they expect to see a big long image. But the "Take a screenshot of the entire page" button doesn't actually work as expected 1) it is actually a screenshot of the current viewport, not the full page 2) the viewport is smaller than it should when DevTools are open and not docked The correct and expected behaviour is the behaviour the GCLI screenshot --fullpage had.
I don't think GCLI did what is described in the summary. I just tried a version of Firefox that still has GCLI, and `screenshot --fullpage` has the same behavior as the button. Eg on a bugzilla page, the scrollbar is on a a subelement, so --fullpage doesn't give any interesting output. That being said, I would love to have a fullpage screenshot tool that is smart enough to simulate a huge viewport and get a real full page screenshot. Maybe something that increases the height of the viewport until scrollbars are either all gone or no longer changing ?
And just for reference, you can try the screenshot button with a page that is scrolled: https://hg.mozilla.org/integration/mozilla-inbound/rev/93516631d63527d285b518f7d5e3b8aa800b627a This will give you a proper full page screenshot (same as gcli --fullpage or :screenshot --fullpage)
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