How to more visibly indicate that a tcolorbox has broken?

Question

I have a tcolorboxenvironment around an AMS theorem that I would like to be breakable. The problem is that I would like a more noticeable indicator for the break than the defaults seem to provide. The best that I've found is to set the option title after break, but my other tcolorboxenvironments don't have the title, so this looks strange.

I've tried setting first's borderline south and last's borderline north, but neither has an effect. I've also tried using first's after upper and last's before lower, also without effect (this outcome would be my preference). I think it should be possible to achieve the borderline effect via the skin engines, but I'm having trouble understanding that much of the documentation.

How can I better indicate that a tcolorbox is continuing onto the next page?

\documentclass{article}

\usepackage{amsthm}
\usepackage[breakable,skins]{tcolorbox}
\usetikzlibrary{decorations.pathmorphing}
\usepackage{lipsum}

\newtheorem{mytheorem}{Theorem}

\tcolorboxenvironment{mytheorem}{
  enhanced,
  colframe=blue,
  interior style={top color=blue!20, bottom color=blue!10},
  breakable=true,
  title after break={Theorem \themytheorem\ Continued}, % works, but not great
  extras first={
    after upper={continued \ldots}, % doesn't work
    borderline south={0.25mm}{0.5mm}{red,decoration={zigzag,amplitude=0.5mm},decorate} % doesn't work
  },
  extras last={
    before lower={\ldots\ continued\par}, % doesn't work
    borderline north={0.25mm}{0.5mm}{red,decoration={zigzag,amplitude=0.5mm},decorate} % doesn't work
  }
}

\begin{document}

\lipsum[1-3]

\begin{mytheorem}[My Theorem]
 \lipsum[4-6]
\end{mytheorem}

\end{document}

Answer 1

This is very similar to this answer. I just added some overlays.

\documentclass{article}

\usepackage{amsthm}
\usepackage[breakable,skins]{tcolorbox}
\usetikzlibrary{decorations.pathmorphing}
\usepackage{lipsum}

\newtheorem{mytheorem}{Theorem}

\tcolorboxenvironment{mytheorem}{
  enhanced,
  colframe=blue,
  interior style={top color=blue!20, bottom color=blue!10},
  breakable=true,
  %title after break={Theorem \themytheorem\ Continued}, % works, but not great
  overlay first={%
      \path[font=\small\itshape] (frame.south) node (cont) {(continued)};
      \draw[thick,red,decoration={zigzag,amplitude=0.5mm},decorate]
       (frame.south west) -- (cont.west)
       (frame.south east) -- (cont.east);
  },
  overlay middle={%
      \path[font=\small\itshape] (frame.south) node {(continued)};
      \draw[thick,red,decoration={zigzag,amplitude=0.5mm},decorate]
       (frame.south west) -- (cont.west)
       (frame.south east) -- (cont.east);
  },
  overlay last={%
      \path[font=\small\itshape] (frame.north) node (cont) {Theorem \themytheorem\ Continued};
      \draw[thick,red,decoration={zigzag,amplitude=0.5mm},decorate]
       (frame.north west) -- (cont.west)
       (frame.north east) -- (cont.east);
  },
}

\begin{document}

\lipsum[1-3]

\begin{mytheorem}[My Theorem]
 \lipsum[4-6]
\end{mytheorem}
\end{document}

One may want to add complete zigzag's that are based on Jake's complete sines, and to make the fill stop at the decoration.

\documentclass{article}

\usepackage{amsthm}
\usepackage[breakable,skins]{tcolorbox}
\usetikzlibrary{decorations.pathmorphing}
\pgfdeclaredecoration{complete zigzag}{initial}{
\state{initial}[
        width=+0pt,
        next state=half up,
        persistent precomputation={\pgfmathsetmacro\matchinglength{
            \pgfdecoratedinputsegmentlength / int(\pgfdecoratedinputsegmentlength/\pgfdecorationsegmentlength)}
            \setlength{\pgfdecorationsegmentlength}{\matchinglength pt}
        }] {}
  \state{half up}[
        width=+.25\pgfdecorationsegmentlength,
        next state=big down]
        {\pgfpathlineto{\pgfqpoint{.25\pgfdecorationsegmentlength}{\pgfdecorationsegmentamplitude}}
    }
  \state{big down}[switch if less than=+.5\pgfdecorationsegmentlength to center finish,
                   width=+.5\pgfdecorationsegmentlength,
                   next state=big up]
  {
    \pgfpathlineto{\pgfqpoint{.5\pgfdecorationsegmentlength}{-\pgfdecorationsegmentamplitude}}
  }
  \state{big up}[switch if less than=+.5\pgfdecorationsegmentlength to center finish,
                 width=+.5\pgfdecorationsegmentlength,
                 next state=big down]
  {
    \pgfpathlineto{\pgfqpoint{.5\pgfdecorationsegmentlength}{\pgfdecorationsegmentamplitude}}
  }
  \state{center finish}[width=0pt, next state=final]{
  }
  \state{final}
  {
    \pgfpathlineto{\pgfpointdecoratedpathlast}
  }
}

\usepackage{lipsum}

\newtheorem{mytheorem}{Theorem}

\tcolorboxenvironment{mytheorem}{
  enhanced,
  colframe=blue,
  interior style={top color=blue!20, bottom color=blue!10},
  breakable=true,
  %title after break={Theorem \themytheorem\ Continued}, % works, but not great
  overlay first={%
      \path[font=\small\itshape] (frame.south) node (cont) {(continued)};
      \begin{scope}[decoration={complete zigzag,amplitude=0.5mm}]
       \path[fill=blue!10]  decorate {([xshift=1.2pt]frame.south west) -- (cont.west)} --++
        (0,0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.south east) -- (cont.east)} --++
        (0,0.5ex) -| cycle;
       \path[fill=white]
        decorate {([xshift=1.2pt]frame.south west) -- (cont.west)} --++
        (0,-0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.south east) -- (cont.east)} --++
        (0,-0.5ex) -| cycle;
       \draw[thick,red,decorate] ([xshift=1.2pt]frame.south west) -- (cont.west);
       \draw[thick,red,decorate] ([xshift=-1.2pt]frame.south east) -- (cont.east);
      \end{scope} 
  },
  overlay middle={%
      \path[font=\small\itshape] (frame.south) node (cont) {(continued)};
      \begin{scope}[decoration={complete zigzag,amplitude=0.5mm}]
       \path[fill=blue!10]  decorate {([xshift=1.2pt]frame.south west) -- (cont.west)} --++
        (0,0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.south east) -- (cont.east)} --++
        (0,0.5ex) -| cycle;
       \path[fill=white]
        decorate {([xshift=1.2pt]frame.south west) -- (cont.west)} --++
        (0,-0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.south east) -- (cont.east)} --++
        (0,-0.5ex) -| cycle;
       \draw[thick,red,decorate] ([xshift=1.2pt]frame.south west) -- (cont.west);
       \draw[thick,red,decorate] ([xshift=-1.2pt]frame.south east) -- (cont.east);
      \end{scope} 
      \path[font=\small\itshape] (frame.north) node (thm) {Theorem \themytheorem\ continued};
      \begin{scope}[decoration={complete zigzag,amplitude=0.5mm}]
       \path[fill=blue!20]  decorate {([xshift=1.2pt]frame.north west) -- (thm.west)} --++
        (0,-0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.north east) -- (thm.east)} --++
        (0,-0.5ex) -| cycle;
       \path[fill=white]
        decorate {([xshift=1.2pt]frame.north west) -- (thm.west)} --++
        (0,0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.north east) -- (thm.east)} --++
        (0,0.5ex) -| cycle;
       \draw[thick,red,decorate] ([xshift=1.2pt]frame.north west) -- (thm.west);
       \draw[thick,red,decorate] ([xshift=-1.2pt]frame.north east) -- (thm.east);
      \end{scope} 
  },
  overlay last={%
      \path[font=\small\itshape] (frame.north) node (thm) {Theorem \themytheorem\ continued};
      \begin{scope}[decoration={complete zigzag,amplitude=0.5mm}]
       \path[fill=blue!20]  decorate {([xshift=1.2pt]frame.north west) -- (thm.west)} --++
        (0,-0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.north east) -- (thm.east)} --++
        (0,-0.5ex) -| cycle;
       \path[fill=white]
        decorate {([xshift=1.2pt]frame.north west) -- (thm.west)} --++
        (0,0.5ex) -| cycle
        decorate {([xshift=-1.2pt]frame.north east) -- (thm.east)} --++
        (0,0.5ex) -| cycle;
       \draw[thick,red,decorate] ([xshift=1.2pt]frame.north west) -- (thm.west);
       \draw[thick,red,decorate] ([xshift=-1.2pt]frame.north east) -- (thm.east);
      \end{scope} 
  },
}

\begin{document}

\lipsum[1-3]

\begin{mytheorem}[My Theorem]
 \lipsum[4-6]
\end{mytheorem}
\end{document}