Re: `@need` badly influences top margin

[Gavin Smith]

On Sat, Aug 20, 2022 at 05:02:44PM +0000, Werner LEMBERG wrote:
> 
> [texinfo.tex version 2022-08-20.16]
> 
> > It wasn't as simple as that but I've made the change to use vertical
> > glue and penalties only (following LaTeX 'needspace').  I've
> > committed it and will upload shortly.
> 
> Alas, it doesn't work well.  It seems that `@need` now unconditionally
> inserts vertical space if not at a page break; see attached example.
> 
> 
>     Werner

Bad interactions among various "clever" parts of code.  This time
it's the code in \safewhatsit that finishes with a large negative
vertical skip.  This leaves it at the correct position on the page,
but this skip is then removed in \dobreak as being less than the
section heading skip, leading to jumping back down the page again
(an example of "two negatives making a positive").

I've committed a fix to hide this negative skip with a penalty.
With this fix, @need does make a very small effect on the page spacing,
with it being tigher without, although I didn't investigate why.
(I suspect a pair of glue immediately followed by negative glue of
the same magnitude has a suppressive effect on overall page stretch
due to stretch in this pair being accounted for.)

This breaks the output from your footnote test case, though - I don't
know why.  All I can suggest there is using a larger value, e.g.
@need 1500.