Horizontal space is free, vertical space is not.

The more lines that are visible on your screen, the less you have to keep in your working memory. Human memory is fragile, so you really don't want to rely on it.

I can only fit 51 lines vertically (damn widescreen laptop) so that one snippet fills a good 1/3 of my screen.

Personally I'd write that as

    if (condition_a) {
        if (condition_b) do_thing_a();
        else do_thing_b();
    }
    else {
        do_something_else();
    }