' ', dowMap[d.getDay()]
      ].join('');
    };

    const canEmbedFile = function f(msg){
      if(!f.$rx){
        f.$rx = /\.((html?)|(txt))$/i;
        f.$specificTypes = [
          'text/plain',
          'text/html'
          // add more as we discover which ones Firefox won't
          // force the user to try to download.
        ];
      }
      if(msg.fmime){
        return (msg.fmime.startsWith("text/")
                || msg.fmime.startsWith("image/"));
        return (msg.fmime.startsWith("image/")
                || f.$specificTypes.indexOf(msg.fmime)>=0);
      }
      return msg.fname && f.$rx.test(msg.fname);
    };

    const adjustIFrameSize = function(msgObj){
      const iframe = msgObj.e.iframe;
      const body = iframe.contentWindow.document.querySelector('body');
      if(body && !body.style.fontSize){
        /** _Attempt_ to force the iframe to inherit the message's text size
            if the body has no explicit size set. On desktop systems
            the size is apparently being inherited in that case, but on mobile
            not. */
        body.style.fontSize = window.getComputedStyle(msgObj.e.content);
      }
      if('' === iframe.style.maxHeight){
        /* Resize iframe height to fit the content. Workaround: if we
           adjust the iframe height while it's hidden then its height
           is 0, so we must briefly unhide it. */
        const isHidden = iframe.classList.contains('hidden');
        if(isHidden) D.removeClass(iframe, 'hidden');
        iframe.style.maxHeight = iframe.style.height
          = iframe.contentWindow.document.documentElement.scrollHeight + 'px';
        if(isHidden) D.addClass(iframe, 'hidden');
      }
    };
    
    cf.prototype = {
      scrollIntoView: function(){
        this.e.content.scrollIntoView();
      },
      setMessage: function(m){
        const ds = this.e.body.dataset;
        ds.timestamp = m.mtime;

                 use case is attached diffs. */
              D.addClass(contentTarget, 'wide');
              const embedTarget = this.e.content;
              const self = this;
              const btnEmbed = D.attr(D.checkbox("1", false), 'id',
                                      'embed-'+ds.msgid);
              const btnLabel = D.label(btnEmbed, "Embed");
              /* Maintenance reminder: do not disable the toggle
                 button while the content is loading because that will
                 cause it to get stuck in disabled mode if the browser
                 decides that loading the content should prompt the
                 user to download it, rather than embed it in the
                 iframe. */
              btnEmbed.addEventListener('change',function(){
                if(self.e.iframe){
                  if(btnEmbed.checked){
                  if(btnEmbed.checked) D.removeClass(self.e.iframe, 'hidden');
                    D.removeClass(self.e.iframe, 'hidden');
                    if(self.e.$iframeLoaded) adjustIFrameSize(self);
                  }
                  else D.addClass(self.e.iframe, 'hidden');
                  return;
                }
                D.disable(btnEmbed);
                const iframe = self.e.iframe = document.createElement('iframe');
                D.append(embedTarget, iframe);                
                D.append(embedTarget, iframe);
                iframe.addEventListener('load', function(){
                  D.enable(btnEmbed);
                  const body = iframe.contentWindow.document.querySelector('body');
                  if(body && !body.style.fontSize){
                    /** _Attempt_ to force the iframe to inherit the message's text size
                        if the body has no explicit size set. On desktop systems
                        the size is apparently being inherited in that case, but on mobile
                        not. */
                    const cs = window.getComputedStyle(self.e.content);
                    body.style.fontSize = cs.fontSize;
                  self.e.$iframeLoaded = true;
                  adjustIFrameSize(self);
                  }
                  iframe.style.maxHeight = iframe.style.height
                    = iframe.contentWindow.document.documentElement.scrollHeight + 'px';
                });
                iframe.setAttribute('src', downloadUri);
              });
              D.append(w, btnEmbed, btnLabel);
            }
            contentTarget.appendChild(w);
          }