how to make text spanner not sticking out to the right

[Werner LEMBERG]

Consider the following snippet.

  \relative c' {
    f4 f f f |
    f4 f f f |
    f4 f f f |
    f4 f f f |
  \once \override TextSpanner.bound-details.left.text = "rit."
  \once \override TextSpanner.bound-details.right.text = "a tempo"
    f4\startTextSpan f f f\stopTextSpan
    f4 f f f |
    f4 f f f |
    f4 f f f |
    f4 f f f |
    f4 f f f |
    f4 f f f |
  }

As can be seen, lilypond (version 2.19.15-1-171-ge86b2ea) makes a very
unfortunate layout decision so that the `a tempo' sticks out to the
right.

For text scripts and lyrics there is the
`Score.PaperColumn.keep-inside-line' property, which avoids similar
effects.  Obviously, this doesn't affect TextSpanner grobs, thus my
question: How can I make the `a tempo' stay horizontally within the
staff?


    Werner