Re: `\textLengthOn` for `TextSpanner` grobs

[Werner LEMBERG]

>> please have a look at this example
>>
>> ```
>> {
>>    \set Score.skipBars = ##t
>>    \override TextSpanner.bound-details.left.text = "poco a poco tempo I"
>>    \override TextSpanner.bound-details.right.text = "Tempo I"
>>    R1*4\startTextSpan |
>>    R1\stopTextSpan
>> }
>> ```
>>
>> What is the equivalent of `\textLengthOn` for `TextSpanner` grobs so
>> that the multi-measure rest gets properly stretched horizontally?
>
> Use
>
>   \override TextSpanner.minimum-length = 30
>   \override TextSpanner.springs-and-rods = #ly:spanner::set-spacing-rods

Thanks.  Alas, doesn't work correctly for broken spanners:

```
\paper {
  line-width = 100\mm
}

{
  \set Score.skipBars = ##t
  \override TextSpanner.bound-details.left.text = "poco a poco tempo I"
  \override TextSpanner.bound-details.left-broken.text = ""
  \override TextSpanner.bound-details.right.text = "Tempo I"
  \override TextSpanner.bound-details.right-broken.text = ""
  \override TextSpanner.minimum-length = 34
  \override TextSpanner.springs-and-rods = #ly:spanner::set-spacing-rods

  R1*4\startTextSpan | \break
  R1*4\stopTextSpan |
  R1 |
}
```

I also wonder whether there is a solution (or should be a solution)
that doesn't need manual entering of a minimum length value that can
only be found by trial and error.  For example, wouldn't it be
sensible to have the possibility of

```
\override TextSpanner.bound-details
            .left.extra-spacing-height = #'(-inf.0 . +inf.0)
\override TextSpanner.bound-details
            .right.extra-spacing-height = #'(-inf.0 . +inf.0)
```

in combination with the `extra-spacing-width` property to get
automatically the correct value(s)?


     Werner