mdframed is causing wrong alignment at the bottom of pages

Question

If you would be so kind to compile this LaTeX document:

\documentclass[a4paper,11pt]{book}

\usepackage[paper=a4paper,twoside=true,lmargin=4cm,
rmargin=4cm,bindingoffset=0cm]{geometry}

\usepackage{mdframed,lipsum,kantlipsum}
\mdfsetup{font=\small}

\begin{document}
\kant[1-8]


\begin{mdframed}
\kant[1-5]
\end{mdframed}

\end{document}

You will find that the bottoms of pages 2 and 3 are misaligned as follows:

Questions:

1. Shouldn't the space between the paragraph and the frame on page 3 expand such that the bottoms align? Why does this not take place?

2. How would you add a flexible (zero, plus something flexible if needed, just like for paragraph spacing) space above the mdframed box too? That would put a less strain on the space between the paragraphs, assuming I resolve question 1 above.

Answer 1

For reasons I don't understand, mdframed outputs the first (split) box in a way that the page bottom will be ragged no matter whether \raggedbottom or \flushbottom (default with book) is configured.

You can get a more normal behaviour with the following patches:

\usepackage{mdframed}

\makeatletter

\patchcmd\mdf@put@frame@i{\hrule \@height\z@ \@width\hsize\vfill}{}{}{}
\patchcmd\mdf@put@frame@i{\hrule \@height\z@ \@width\hsize\vfill}{}{}{}
\patchcmd\mdf@put@frame@i{\hrule \@height\z@ \@width\hsize\vfill}{}{}{}

\makeatother

As I really don't know what this is supposed to achieve, this will probably break something, but at least, the page bottom is aligned now ;-)

The space above the mdframed box is configured with skipabove, so you get flexible glue by

\mdfsetup{font=\small,skipabove=\parskip}