`toprule=0pt` changes the layout of the left-bottom and right-bottom corners

[dbitouze]

As shown by the following MCE, toprule=0pt changes the layout of the left-bottom and right-bottom corners. Is it a bug or a feature?

\documentclass{article}
\usepackage{tcolorbox}
\usepackage[paperwidth=12cm,paperheight=2cm,margin=.25cm]{geometry}
\pagestyle{empty}
\begin{document}
\tcbset{sharp corners=north}
\begin{tcolorbox}
  Foo
\end{tcolorbox}
\newpage
\begin{tcolorbox}[toprule=0pt]
  Foo
\end{tcolorbox}
\end{document}

[T-F-S]

Actually, it is a feature. With rounded corners, a tcolorbox has four identical inner arcs (interior) and outer arcs (frame). The radius of an outer arc is the radius of the inner arc plus the rule width.

Things get complicated, if there is no identical rule width. Here is valid:

• outer arc radius = inner arc radius + minimum of the rules for the four sides

Your toprule=0pt therefore makes the outer arc radius smaller.

Fortunately, you can overrule this algorithm by setting a manual outer arc radius. With default values for rule 0.5mm plus inner arc radius 1.0mm you may use outer arc=1.5mm.

\documentclass{article}
\usepackage{tcolorbox}
\usepackage[paperwidth=12cm,paperheight=2cm,margin=.25cm]{geometry}
\pagestyle{empty}
\begin{document}

\begin{tcolorbox}
  Foo
\end{tcolorbox}
\newpage
\begin{tcolorbox}[toprule=0pt]
  Foo
\end{tcolorbox}
\newpage
\begin{tcolorbox}[toprule=0pt,outer arc=1.5mm]
  Foo
\end{tcolorbox}

\newpage
\tcbset{sharp corners=north}
\begin{tcolorbox}
  Foo
\end{tcolorbox}
\newpage
\begin{tcolorbox}[toprule=0pt]
  Foo
\end{tcolorbox}
\newpage
\begin{tcolorbox}[toprule=0pt,outer arc=1.5mm]
  Foo
\end{tcolorbox}
\end{document}

Box 2 is the intended behaviour for rounded corners, Box 3 has the manual settings for demonstration.

Box 6 is the box you possibly want to use.

[dbitouze]

OK, thanks. Too bad the feature isn't to autocorrect the outer arc radius :wink:

[muzimuzhi]

skin=enhancedlast gives a similar result, but the box is higher than the one with toprule=0pt by width of toprule.

[muzimuzhi]

Things get complicated, if there is no identical rule width. Here is valid:

• outer arc radius = inner arc radius + minimum of the rules for the four sides

Maybe this can be added to the doc of /tcb/auto outer arc. Its current doc only says the auto-computed outer arc is dependent on /tcb/inner arc. https://github.com/T-F-S/tcolorbox/blob/067f384e2ba85ba3fd91958eb6adb8a1bcea6c29/doc/latex/tcolorbox/tcolorbox.doc.coreoptions.tex#L1169-L1172

---

If there's a way to expandably retrieve current values of aspects of tcolorboxes set by options, then the magic number 1.5mm in outer arc=1.5mm can be replaced with an expression, like outer arc/.expanded=\dimeval{\tcbgetvalue{outer arc}-\tcbgetvalue{bottomrule}}.

Example below shows various workarounds, including use of a new option fix outer arc which immediately computes radius of outer arc in the way auto outer arc will do, but in a latter place.

Boxes a.3.x and b.2.x all give the expected result, but in different ways.

Example

```tex \documentclass{article} \usepackage[skins]{tcolorbox} \usepackage[hmargin=.5in, twocolumn]{geometry} \pagestyle{empty} \makeatletter \tcbset{ fix outer arc/.code={% \tcb@comp@arc@auto % compute \tcb@outer@arc right now \let\tcb@comp@arc=\@empty % no further computation is needed } } \makeatother \NewDocumentCommand{\testTcb}{ O{} m }{ \begin{tcolorbox}[#1] \vphantom{Foo}\smash{#2} \end{tcolorbox} } \begin{document} \tcbset{every box/.style={sharp corners=north}} \testTcb[] {a.1 \texttt{sharp corners=north}} \testTcb[toprule=0pt] {a.2 \texttt{+ toprule=0pt}, smaller outer arc} \testTcb[toprule=0pt, outer arc=1.5mm] {a.3.1 \texttt{+ outer arc=1.5mm}} \makeatletter \testTcb[ outer arc=1.5mm, % evaluates to 1mm + .5mm = 1.5mm outer arc/.expanded=\dimeval{\kvtcb@arc+(\kvtcb@top@rule@stand)}, toprule=0pt ]{a.3.2 \texttt{+ outer arc = arc - original toprule}} \makeatother \testTcb[fix outer arc, toprule=0pt] {a.3.3 \texttt{+ fix outer arc} (new option)} \newpage \tcbset{every box/.style={skin=enhancedlast}} \testTcb[] {b.1 \texttt{skin=enhancedlast}, box higher by toprule} \testTcb[top=1.5mm] {b.2.1 \texttt{+ top=1.5mm}} \makeatletter \testTcb[ top/.expanded=\dimeval{\kvtcb@top-(\kvtcb@top@rule@stand)} ]{b.2.2 \texttt{+ top = top - toprule}} \makeatother \end{document} ```