Color background of chapter headings
I'm trying to change the background color of the chapter headings. I'd
like the full top part of the page, including top and right margins to
turn gray until the titlerule.
Currently they look like this:
\documentclass[10pt,twoside,openright]{book}
\usepackage{titlesec}
\titleformat{\chapter}[display]{\LARGE\sffamily\bfseries\raggedright}
{\leftskip2cm {\Large\textmd{\chaptertitlename~\thechapter}}}
{0cm}{\leftskip2cm}[\vspace{4mm} {\color{dgray}\titlerule[.6mm]}
\enlargethispage{-\parskip}]
The \enlargethispage is used to make some extra space for the title
number, which appears at the bottom of the page only at the first page of
the chapter. My attempts with using the framed environment or colorbox
have so far not been successful.