🖨️ Some PDF layout improvements

[fonsp]

Header positions

We now avoid a widow header, so there is a higher likelihood that a header will appear on the same page as the content it is a header to:

Screenshots are before & after

The level 1 header will always create a new page

Other page break changes

The <br> or --- horizontal rule will now act like a page break when creating a PDF

Before this PR, Pluto avoids all page breaks inside of a pluto-cell. I.e. if a cell (content + code (if not folded)) needs to be fragmented between pages, it will be moved to the next page.

This PR adds an exception for "prose cells": cells that contain mostly text. This means that cells that just contain a couple paragraphs are allowed to break inside. Here, you expect breaks inside, and it creates a less fragmented document overall.

Avoid empty final page

Made some changes that were sometimes causing an extra empty page at the end of the document.

Full before/after comparisons

I printed all the featured notebooks before and after:

before.zip

after.zip

[github-actions]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/fonsp/Pluto.jl", rev="print-stylesheet-improvements")
julia> using Pluto