documentclass{book}

usepackage[most]{tcolorbox}
usepackage{varwidth}
tcbuselibrary{skins,breakable}

usepackage{polyglossia}
setmainlanguage{english}

newcommand{myboxnode}[2]{%
node[fill=#2!5!white, draw=#2!75!black, line width=.5pt,
    below right, minimum height=2em,
   text width=.8cm, align=center,
   font=bfseries] at ([xshift=3mm,yshift=-1mm]frame.north east) {#1 thetcbcounter}}

tcbset{
    mystyle/.style 2 args={
        enhanced jigsaw, breakable,
        colback=white, colframe=white,
        sharp corners,
        boxrule = 0mm,
        top=2mm, bottom=2mm, left=2mm, right=2mm,
%       varwidth boxed title,
        tikznode boxed title={minimum width=textwidth/4-15mm-4.5mm},
        attach boxed title to top right={%
            xshift=.5mm,yshift=- tcboxedtitleheight+.5mm},
        boxed title style={%
            enhanced,
            sharp corners,
            colframe=#1!75!black,
            left=1mm, right=1mm,
            boxrule=.5mm},
        colbacktitle=#1!5!white,
        coltitle=black,
        extrude right by=-15mm,
        underlay unbroken and first={%
            myboxnode{#2}{#1};
            draw [#1!75!black,line width=.5mm]([shift={(15mm-.5pgflinewidth,-15mm+.5pgflinewidth)}]frame.north east)|-([shift={(-textwidth/4+15mm,+.5pgflinewidth)}]frame.north east);
            draw [#1!75!black,line width=.5mm]([shift={(.5pgflinewidth,-15mm+.5pgflinewidth)}]frame.north west)|-([shift={(textwidth/4,+.5pgflinewidth)}]frame.north west);        },
        underlay unbroken and last={%
            draw [#1!75!black,line width=.5mm]([yshift=.25mm]frame.south west)(textwidth/4,0)--(0,0);
            draw [#1!75!black,line width=.5mm]([xshift=.25mm,yshift=.25mm]frame.south west)--+(0,15mm);
            draw [#1!75!black,line width=.5mm]([xshift=14.75mm,yshift=.25mm]frame.south east)--+(0,15mm);
            draw [#1!75!black,line width=.5mm]([yshift=.25mm]frame.south east)(3*textwidth/4,0)--(textwidth,0);
        },
    },
    mytitle/.style 2 args={
        code={%
            ifstrempty{#1}
            {tcbset{
                before upper={parshape 4
                    0pt dimexpr hsize-14mmrelax
                    0pt dimexpr hsize-14mmrelax
                    0pt dimexpr hsize-14mmrelax
                    0pt hsize}}}
            {tcbset{
                title={#1},
                before upper={vspace*{baselineskip}parshape 3
                    0pt dimexpr hsize-14mmrelax
                    0pt dimexpr hsize-14mmrelax
                    0pt hsize},
                underlay unbroken and last={
                    draw [#2!75!black,line width=.5mm](frame.south east)--++(180:tcboxedtitlewidth-pgflinewidth);
                }}}
        },
    }
}

newtcolorbox[auto counter]{proposition}[2][]{%
    mystyle={blue}{Prop},
    mytitle={#2}{blue},
    #1}

newtcolorbox[auto counter]{definition}[2][]{%
    mystyle={green}{Def},
    mytitle={#2}{green},
    #1}

begin{document}
pagestyle{empty}

begin{definition}{}
A non-titled definition
vspace{2.2cm}
end{definition}

begin{proposition}{A long title}
A titled proposition
vspace{2cm}
end{proposition}

begin{definition}[height=2cm,label=Exlab]{A}
A titled definition
end{definition}
ref{Exlab}
end{document}

tcbsetmacrotoheightofnodemyheight{myboxnode{#2}{#1}},
    underlay unbroken and first={%
        myboxnode{#2}{#1};
        draw [#1!75!black,line width=.5mm]([shift={(15mm-.5pgflinewidth,-dimexprmyheight+.5pgflinewidth)}]frame.north east)|-([shift={(-textwidth/4+15mm,+.5pgflinewidth)}]frame.north east);
        draw [#1!75!black,line width=.5mm]([shift={(.5pgflinewidth,-dimexprmyheight+.5pgflinewidth)}]frame.north west)|-([shift={(textwidth/4,+.5pgflinewidth)}]frame.north west);        },
    underlay unbroken and last={%
        draw [#1!75!black,line width=.5mm]([yshift=.25mm]frame.south west)(textwidth/4,0)--(0,0);
        draw [#1!75!black,line width=.5mm]([xshift=.25mm,yshift=.25mm]frame.south west)--+(0,dimexprmyheight);
        draw [#1!75!black,line width=.5mm]([xshift=14.75mm,yshift=.25mm]frame.south east)--+(0,dimexprmyheight);
        draw [#1!75!black,line width=.5mm]([yshift=.25mm]frame.south east)(3*textwidth/4,0)--(textwidth,0);
    },