[texhax] \vskip at beginning of file

[Jim Diamond]

On Tue, Jul 19, 2016 at 12:21 (+0100), David Carlisle wrote:
> On 19 July 2016 at 12:02, Philip Taylor <p.taylor at rhul.ac.uk> wrote:
>> Rodolfo Medina wrote:

>>> Is it possible to avoid that ugly trick, and how?

>> \leavevmode \vskip ... is one way, \topglue ... is another.
>> Philip Taylor

> The \topglue macro is certainly the best of these options.

<snip>

> \topglue uses \hrule height 0pt as the basic node to prevent the skip
> being discarded but takes care not to disturb \prevdepth, and corrects
> for \topskip.

In the context of someone putting some space at the top of the page,
can anyone explain to me why we care about preserving \prevdepth ?

(Or is this only significant if \prevdepth is "large" and the white
space at the top is "small"?)

Thanks.

                                Jim