vertical spacing of top-level `\markup` and `\bookpart`

[Werner LEMBERG]

Consider this example.

```
\paper {
  markup-markup-spacing.stretchability = 1000
  tagline = ##f
}

\markup "top"
\markup "middle"
\markup "bottom"

\pageBreak

%\bookpart {
  { c' }

  \markup "top"
  \markup "middle"
  \markup "bottom"
%}
```

If I activate the `\bookpart` block, the stretchability is completely
lost – both inside and outside of `\bookpart`.  Is this expected?  How
can I control the stretchability for global markups section-wise
(e.g., in `\bookparts`)?


   Werner