Then just hold the right click button, and drag down a tad. It's amazingly simple and easy. And that instantly opens a new tab.

You can do it anywhere on the web page and it works. It's easier than having to do a double click from the tab bar, because then you have to be on the tab bar.

When the window is not maximized, the tab bar is clearly separated from the "windows status bar". So having a different action between the tab bar (new tab) and the top status bar (maximize the window) would not be "less" native.

When the window is maximized, the behaviour is already "not" native because the status bar is used as a tab bar, which is not the standard Windows behaviour. So adding specific actions related to this "un-standard" behaviour seems to be quite relevant to me.
Moreover, there is this "1 pixel" space at the top of the windows, which I find very clever and very useful: it could still be used to maximize the window on Opera 12.

Maybe the real reason is the fact that it may not be as easy with Chromium than what it was in the past. Anyway the status is the same: it is not possible anymore.

And that's a shame, beacause I think it is really very frustrating to be forced to aim and click this tiny "+" on the tab bar... Double click was such a good idea!

At the moment, there's no way to bring this feature back, as far as I know.

This question has already been asked multiple times, but they never really answered (at least to my knowledge).

The only thing I remember is that they made a poll months ago, and they were asking people what was their favourite way to open a new tab. Double click was in the top results list, so there may be something in the future. It's been while, though, and no news... Sadly.