Options:

     .copyFromElement: DOM element

     .copyFromId: DOM element ID




     .extractText: optional callback which is triggered when the copy
     button is clicked. It must return the text to copy to the
     clipboard. The default is to extract it from the copy-from
     element, using its [value] member, if it has one, else its
     [innerText]. A client-provided callback may use any data source
     it likes, so long as it's synchronous. If this function returns a
     falsy value then the clipboard is not modified. This function is
     called with the fully expanded/resolved options object as its
     "this" (that's a different instance than the one passed to this
     function!).

     At least one of copyFromElement, copyFromId, or extractText must
     be provided, but if copyFromId is not set and e.dataset.copyFromId
     is then that value is used in its place. extractText() trumps the
     other two options.

     .cssClass: optional CSS class, or list of classes, to apply to e.

     .style: optional object of properties to copy directly into
     e.style.

     .oncopy: an optional callback function which is added as an event

      }else{
        e = this.getMessageElemById(id);
      }
      if(!e || !id) return false;
      else if(e.$isToggling) return;
      e.$isToggling = true;
      const content = e.querySelector('.content-target');
      if(!content){
        console.warn("Should not be possible: trying to toggle text",
                     "mode of a message with no .content-target.", e);
        return;
      }
      if(!content.$elems){
        content.$elems = [
          content.firstElementChild, // parsed elem
          undefined // plaintext elem
        ];
      }else if(content.$elems[1]){
        // We have both content types. Simply toggle them.
        const child = (
          content.firstElementChild===content.$elems[0]
            ? content.$elems[1]
            : content.$elems[0]
        );
        D.clearElement(content);
        if(child===content.$elems[1]){
          /* When showing the unformatted version, inject a
             copy-to-clipboard button. This is a workaround for
             mouse-copying from that field collecting twice as many
             newlines as it should (for unknown reasons). */
          const cpId = 'copy-to-clipboard-'+id;
          /* ^^^ copy button element ID, needed for LABEL element
             pairing.  Recall that we destroy all child elements of
             `content` each time we hit this block, so we can reuse
             that element ID on subsequent toggles. */
          const btnCp = D.attr(D.addClass(D.span(),'copy-button'), 'id', cpId);
          F.copyButton(btnCp, {extractText: ()=>child._xmsgRaw});
          const lblCp = D.label(cpId, "Copy unformatted text");
          lblCp.addEventListener('click',()=>btnCp.click(), false);
          D.append(content, D.append(D.addClass(D.span(), 'nobr'), btnCp, lblCp));
        }
        delete e.$isToggling;
        D.append(content, child);
        return;
      }
      // We need to fetch the plain-text version...
      const self = this;
      F.fetch('chat-fetch-one',{
        urlParams:{ name: id, raw: true},
        responseType: 'json',
        onload: function(msg){
          content.$elems[1] = D.append(D.pre(),msg.xmsg);
          content.$elems[1]._xmsgRaw = msg.xmsg/*used for copy-to-clipboard feature*/;
          self.toggleTextMode(e);
        },
        aftersend:function(){
          delete e.$isToggling;
          Chat.ajaxEnd();
        }
      });

                    e = n;
                  }
                  eMsg.scrollIntoView();
                }
              ));
              const toolbar2 = D.addClass(D.div(), 'toolbar');
              D.append(this.e, toolbar2);
              if(eMsg.querySelector('.content-target')){
                /* ^^^ messages with only an embedded image have no
                   .content-target area. */
                D.append(toolbar2, D.button(
                  "Toggle text mode", function(){
                    self.hide();
                    Chat.toggleTextMode(eMsg);
                  }));
              }
              if(eMsg.dataset.xfrom){
                /* Add a link to the /timeline filtered on this user. */
                const timelineLink = D.attr(
                  D.a(F.repoUrl('timeline',{
                    u: eMsg.dataset.xfrom,
                    y: 'a'
                  }), "User's Timeline"),