tcolorbox overlays and space needed

Question

I'm moving from mdframed to using tcolorbox, and I'm almost where I want to be.

Using mdframed, I can use the singleextra and firstextra parameters to add nodes to my frame, and this changes the height of the frame as a whole (i.e. if my new node sticks out above the frame, the height of the entire thing is increased; ditto if my new node sticks out below the frame). This makes my life easy.

Using tcolorbox, I can use the overlay parameter to add nodes to the frame, but the size does not adjust automatically. There is, if you include \tcbuselibrary{hooks,skins} (not sure which one exactly does it) an enlarge top by parameter that will add space needed so thew node that sticks out above the frame doesn't overlap previous content.

The MWE below shows the use of an overlay and enlarge top by.

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{skins,hooks}
\usepackage{lipsum}

\tcbset{
  frogbox/.style={
    enhanced,
    colback=green!10,
    colframe=green!65!black,
    enlarge top by=5.5mm,
    overlay={
      \foreach \x in {2cm,3.5cm} {
        \begin{scope}[shift={([xshift=\x]frame.north west)}]
        \path[draw=green!65!black,fill=green!10,line width=1mm] (0,0) arc (0:180:5mm);
        \path[fill=black] (-0.2,0) arc (0:180:1mm);
        \end{scope}
      }
    }
  }
}

\tcbset{
  app/.style={
    overlay app={%
      \draw(frame.north)--(frame.south);
    }
  }
}

\begin{document}

\lipsum[1]
\begin{tcolorbox}[frogbox,title=My title]
This is a \textbf{tcolorbox}.
\end{tcolorbox}

\lipsum[2]
\begin{tcolorbox}[frogbox,app,title=My title]
This is a \textbf{tcolorbox}.\par
Here, we apply a second overlay.
\end{tcolorbox}
\end{document}

Because of the enlarge top by, the eyes don't overlap the text above the frogbox. Remove the enlarge top by, and the eyes do overlap the text above the frogbox.

That all said, is there an alternative to overlay that will account for potentially increased size of the frame+nodes? Is there an easier way to manage this than examining every node that might be added [it's not a static list for every frame of this type, they're markers regarding meta information] to see if the space allocated for the frame needs to be increased?

I could simply assume the new nodes will be present and allocate space for them, and it does look kind of nice to see the frames aligning at the top of the page whether the extra nodes are present or not (my current strategy), but I'd like to explore a more sophisticated presentation, where the extra space is present or not depending on whether the nodes are present. What's an easy way to do this?

Answer 1

I experimented a little bit how to achieve the return to the bounding box tracking of tikz. The solution is quite simple. If you add

\tikzset{overlay=false}

to your overlay code, the tracking is re-established.

Note: No effect in tcolorbox/TikZ that ships with TeX Live 2013. Updating to TeX Live 2106 fixes. Stay current! --kjd

The full code is:

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{skins,hooks}
\usepackage{lipsum}

\tcbset{
  frogbox/.style={
    enhanced,
    colback=green!10,
    colframe=green!65!black,
    enlarge top by=5.5mm,
    overlay={
      \tikzset{overlay=false}%< -----------------------
      \foreach \x in {2cm,3.5cm} {
        \begin{scope}[shift={([xshift=\x]frame.north west)}]
        \path[draw=green!65!black,fill=green!10,line width=1mm] (0,0) arc (0:180:5mm);
        \path[fill=black] (-0.2,0) arc (0:180:1mm);
        \end{scope}
      }
    }
  }
}

\tcbset{
  app/.style={
    overlay app={%
      \draw([yshift=2cm]frame.north)--(frame.south);
    }
  }
}

\begin{document}

\lipsum[1]
\begin{tcolorbox}[frogbox,title=My title]
This is a \textbf{tcolorbox}.
\end{tcolorbox}

\lipsum[2]
\begin{tcolorbox}[frogbox,app,title=My title]
This is a \textbf{tcolorbox}.\par
Here, we apply a second overlay.
\end{tcolorbox}
\end{document}

Please, note that I do not expect that this will do any good for breakable boxes. The calculated size will not be identical to the real size and breaking will not look correct.

For unbreakable boxes, this should work, but will interfere with bounding box settings like enlarge top by etc.