Switching format keeps the original quote, not a flattened copy (#409) (#409)

Switching a reply between plain text and rich text converted whatever
body the draft was showing. Going from plain text to rich, that meant
the quoted message came back as the "> " text quote run through a
converter -- the sender's formatting, images and links gone, even though
the original markup was sitting on the draft untouched.

Both forms of the quote are prepared when the reply opens, so keep them
on the draft and re-attach the right one when the format changes. Only
what the author typed above the quote is converted. Where the quote
can't be found any more -- edited by hand, or a draft that quotes
nothing -- the whole body is converted as before, which is what every
non-reply draft does.

No new strings.

(cherry picked from commit 88f9e6c50a04f8ffc4702d1d1e3cffa6a93e7690)

web/src/views/compose/Composer.tsx

@@ -147,12 +147,26 @@ export function Composer({ draft }: { draft: Draft }) {
     patch({ sendAt: at.getTime() });
   };
 
+  /*
+   * Switching format converts what has been written, but the quoted message
+   * is not something this draft wrote: it was prepared in both formats when
+   * the reply opened. Converting the plain-text quote into HTML would hand
+   * back a flattened copy of a message that still exists in its original
+   * markup, so re-attach that instead, and keep only what the author typed
+   * above it. Where the quote can no longer be found -- edited, or a draft
+   * that quotes nothing -- convert the whole body as before.
+   */
   const toggleFormat = () => {
     // Whichever way the format is changed, the offer has been answered.
     if (d.format === "html") {
-      patch({ format: "text", text: htmlToText(d.html), formatOffer: null });
+      const at = d.quoteHtml ? d.html.indexOf('<div class="ihm-quote">') : -1;
+      const written = at >= 0 ? htmlToText(d.html.slice(0, at)) : htmlToText(d.html);
+      patch({ format: "text", text: at >= 0 ? written.replace(/\s+$/, "") + d.quoteText : written, formatOffer: null });
     } else {
-      patch({ format: "html", html: textToHtml(d.text, { linkify: false, quoteColors: false }).replace(/\n/g, "<br>"), formatOffer: null });
+      const keeps = Boolean(d.quoteText) && d.text.endsWith(d.quoteText);
+      const written = keeps ? d.text.slice(0, d.text.length - d.quoteText.length) : d.text;
+      const asHtml = textToHtml(written, { linkify: false, quoteColors: false }).replace(/\n/g, "<br>");
+      patch({ format: "html", html: keeps ? asHtml + d.quoteHtml : asHtml, formatOffer: null });
     }
   };