I have seen few search results for this, so I thought it would be a good idea to share what I found out. By full-page screenshot I mean a screenshot that shows the page as if the viewport would fit the whole content.
I thought this would need extensions (of which all are nonfree, and I'm not trusting nonfree extensions for obvious reasons), but it looks like Chromium has had it all this time. It's not as beautiful as some other browsers, but it works.
Open the Developer Tools on the page you want to screenshot.
Make sure theElements pane is visible.
In that pane, select the
<html>opening tag, right-click and choose 'Capture node screenshot'.Save the resulting image where you want.
This also works for any other HTML element, of course.