LaTeX wraps the page box with an additional `\vbox` to add the margins to the box being shipped out (so it does not use `\hoffset`).

From inside LaTeX's `pre_shipout_filter` callback, is there an easy way to determine the box of the page itself and/or the offset use for it (for margins, i.e. `\@themargin` used when the shipout box is boxed)?

Usually naively searching for the first vlist works, but it is not good enough as boxes might have been added in the shipout hooks, and from what I can tell `\@themargin` is set locally.

Currenlty I mark the page box with an attribute by patching `\@outputpage`, but I prefer to use a different method.