empty layout height does not match height with text

[cmyr]

This is a problem on at least cairo and macOS, and is most obvious on cairo.

[richard-uk1]

Driveby comment: I'm sure this is the same in the browser: if you have a <span> with nothing in it, it has height 0 whereas as soon as it has a letter in it the height is 1 line height.

[cmyr]

Yea, this is really specifically about text boxes; when they're empty we still want to know how tall the text would be if it existed, so that we can correctly size the box.