Finder actually fills the screen (the Apple app I did originally test since it used to represent reference windowing behavior) but only in list mode, which happened to be my default. In icon mode it does the old school top-to-bottom thing, as I found out after the replies.
But Chrome fills the screen, OmniFocus fills, Keyboard Maestro's editor fills, Firefox fills, etc. I think a lot of third party apps will fill the screen since it seems to come down to whether the dev gives it a different aspect ratio than native to "show all content." Most stuff seems not to do so anymore, presumably because that behavior is weird.
But to your point, apparently Safari was a huge oversight on my part and it's definitely not as consistent as I thought it was. Sorry if I got hopes up.
That said, to the extent an app will do that at all, opt-click-max or title bar double-click is the way to do it. Otherwise you're stuck with one of the screen snap apps that can also redefine the buttons for you.