[mg]

That's a cool idea.

So a simple return makes a newline, and two quick returns have a different meaning. I like that.

The way I support multiline is via the little triangle in the search bar. I think I'll keep it, because it is easier to discover. The double-return would need some kind of textual hint. Like when you press enter once, a little hint could pop up below the search area telling you that to leave the text entry, you can hit enter twice.

And on mobile I rarely use multiline searches I think. On the Desktop I use shift-return to create a newline.