problem with vertical spacing paper variables

[Werner LEMBERG]

I'm working on integrating the visual grob index into the NR.  To do
that, I'm trying to get the largest possible `@image` without overfull
warnings, then setting the margins to zero.  This works just fine.

However, when also setting the vertical spacing paper variables to
zero to make the ink directly touch the margins, I still get some
additional vertical space at the very top starting on the second page
that I can't explain.  It's the '0.51' value showing up in upper right
corner of the annotated example code.  What is this?  How can I
suppress this?

Here's my test code.

```
#(set-default-paper-size '(cons (* 160 mm) (* (+ 673.2 4) pt)))

{ \repeat unfold 440 f'2 }


\paper {
  left-margin = 0
  right-margin = 0
  top-margin = 0
  bottom-margin = 0

  oddHeaderMarkup = ##f
  evenHeaderMarkup = ##f
  oddFooterMarkup = ##f
  evenFooterMarkup = ##f

  ragged-bottom = ##t
  ragged-last-bottom = ##t

  annotate-spacing = ##t

  top-system-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))
  top-markup-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))
  last-bottom-spacing =
    #'((basic-distance . 0)
       (minimum-distance . 0)
       (padding . 0)
       (stretchability . 0))
}
```


    Werner

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