Thanks, that's exactly what I needed. I got inspired by your post at https://adityam.github.io/context-blog/post/frame-with-solid-left-line/ for my original code. I just added linecap:=butt; at the beginning of the metapost code to get squared ends. Gilles
Framed does not provide enough hooks to control the spaces. For something like this, you can also use metapost to draw the background:
\showframe \showboxes \defineframedtext[leftbartext][ width=broad, frame=off, rulethickness=.5ex, background=mp:offsetframe, offset=overlay, ]
\defineoverlay[mp:offsetframe][\useMPgraphic{mp:offsetframe}]
\startuseMPgraphic{mp:offsetframe} linecap:=butt; draw leftboundary OverlayBox xshifted -(LeftMarginDistance + 0.5OverlayLineWidth) withpen pencircle scaled OverlayLineWidth; setbounds currentpicture to OverlayBox; \stopuseMPgraphic
\starttext \startleftbartext \input knuth \stopleftbartext \stoptext
Aditya