Taken from nobody knows shoes:


```
One Quick Note About Widths
Widths may be a positive or negative number of pixels.
stack :width => 80 do; end
stack :width => -80 do; end
You’ll often see this. The first stack is 80 pixels wide. And
the second is 100% minus 80 pixels wide. So, together they
are 100%, they fill their parent box.
```

I'll open an issue.