How to make a tcolobox stick the bottom of the page ? (and why \vfill is not perfectly working?)

Question

I have created a MWE where one can picture my issue.

\documentclass[12pt]{article}
\usepackage[
    left   =0cm,
    right  =0cm,
    top    =0cm,
    bottom =0cm,
    paperwidth  = 521pt,
    paperheight = 384pt
    ]{geometry}
\usepackage{lipsum}
\usepackage[skins, breakable, theorems]{tcolorbox}

\pagestyle{empty}

\newtcolorbox{Mybox}{
    %enhanced,
    arc = 0pt,
    boxrule = 0pt,
    colback = blue,
    colupper = white,
    top= 2pt,
    bottom = 1pt
}


\begin{document}

\begin{Mybox}
    Some small upper text.
\end{Mybox}

\lipsum[1]

\vspace*{\fill}%
\begin{Mybox}
    Some small lower text.
\end{Mybox}

\end{document}

In the previous image, one can see that the top tcolorbox sticks perfectly to the top but the bottom one is not (quite). How can I fix this? Note the unusual geometry.

The geometry package does not show any other spacing than bottom so I should be able to reach the bottom of the page. Is that a flaw of the \vfill then?

PS: I do not want to work with fancyhdr here since I want something far more simple.

Answer 1

My issue was actually caused by a spacing specific to tcolorbox (that one can find in their documentation, page 11). Therefore, to solve my issue one just need to add after=0pt as option for the tcolorbox.

\documentclass[12pt]{article}
\usepackage[
    left   =0cm,
    right  =0cm,
    top    =0cm,
    bottom =0cm,
    paperwidth  = 521pt,
    paperheight = 384pt
    ]{geometry}
\usepackage{lipsum}
\usepackage[skins, breakable, theorems]{tcolorbox}

\pagestyle{empty}

\newtcolorbox{Mybox}{
    %enhanced,
    arc = 0pt,
    boxrule = 0pt,
    colback = blue,
    colupper = white,
    top= 2pt,
    bottom = 1pt,
    after = 0pt
}


\begin{document}

\begin{Mybox}
    Some small upper text.
\end{Mybox}

\lipsum[1]

\vfill%
\begin{Mybox}
    Some small lower text.
\end{Mybox}

\end{document}

I also changed \vspace*{\fill} to \vfill, as suggested by Fran.