% !TEX program = xelatex

documentclass{scrbook}

usepackage{wrapfig}
usepackage{graphicx}
usepackage{tcolorbox}

setlength{parskip}{3mm}
setlength{parindent}{0pt}

newtcolorbox[auto counter]{sidebar}[2][]{
    parbox=false, % normal paragraph spacing (unless wrapped!)
    sharp corners,
    colframe=black,
    colback=black!15!white,
    title=#2,
    #1
}

begin{document}

begin{sidebar}{Test without wrapping}
    Paragraph 1.

    Paragraph 2.
end{sidebar}

begin{wrapfigure}{r}{0.5linewidth}
    begin{sidebar}{Test with wrapping and no explicit parksip}
        Paragraph 1.

        Paragraph 2.
    end{sidebar}
end{wrapfigure}

begin{wrapfigure}{r}{0.5linewidth}
    begin{sidebar}{Test with wrapping and explicit parskip}
        setlength{parskip}{3mm}setlength{parindent}{0pt}Paragraph 1.

        Paragraph 2.
    end{sidebar}
end{wrapfigure}

end{document}