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I am using tcolorbox to draw boxes around equations. Since the box widths are equal to the equation widths, long titles are often shrunk, hence looking ugly, as shown here:

Is there any way to draw a box of auto-width around the equation and title?

A minimal working example is here:

documentclass[11pt]{article}

usepackage[many]{tcolorbox}



begin{document}

begin{equation}

tcbhighmath[title=A long title hence does not fit]{lambda=at^2}

end{equation}



begin{equation}

tcbhighmath[title=short-title]{lambda=at^2+bt^3}

end{equation}

end{document}

A different approach by defining a new key autowidth title that set the minimum text width to be equal to the the title length

documentclass[11pt]{article}

usepackage[many]{tcolorbox}

pgfkeysdef{/tcb/autowidth title}{

pgfmathwidth{"#1"}

defwidth{pgfmathresult pt}

tcbset{title=#1,

tcbox width=minimum center,

text width=width}}



begin{document}

begin{equation}

tcbhighmath[autowidth title=A long title hence does not fit ]{lambda=at^2}

end{equation}



begin{equation}

tcbhighmath[autowidth title=short-title]{lambda=at^2+bt^3}

end{equation}









end{document}

From this excellent macro of Thomas F. Sturm: ....fit the width...., here there is my adapt answer:

documentclass[11pt]{article}

usepackage[many]{tcolorbox}

tcbset{longtitle/.style={%

  title={#1},

  before upper={begin{tabular}{@{}l@{}l}phantom{#1}\[thedimexpr-htstrutbox-dpstrutbox]},

  after upper={end{tabular}}}

  }



begin{document}

begin{equation}

tcbhighmath[longtitle=A long title hence does now fit]{$lambda=at^2$}%<---- for a double dollar

end{equation}





begin{equation}

tcbhighmath[title=short-title]{lambda=at^2+bt^3}%<---- without a double dollar

end{equation}

end{document}

A fix for avoiding the $ characters in the main argument:

documentclass[11pt]{article}

usepackage{array}

usepackage[many]{tcolorbox}



tcbset{longtitle/.style={%

  title={#1},

  before upper={%

    $begin{array}{@{}>{displaystyle}l@{}}

    mbox{phantom{#1}}\[thedimexpr-htstrutbox-dpstrutbox]%

  },

  after upper={end{array}$}},

}



begin{document}



begin{equation}

tcbhighmath[longtitle=A long title hence does now fit]{lambda=at^2}

end{equation}



begin{equation}

tcbhighmath[title=short-title]{lambda=at^2+bt^3}                              

end{equation}



end{document}