Mail: moving between messages no longer rebuilds the whole page (only the reader and the selected row change)

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/mail.js

@@ -138,6 +138,15 @@ async function mailAdminCard() {
 async function viewMail(args = []) {
   const v = $('#view');
   if (!cfg.mail) { v.innerHTML = '<div class="wrap"><h1>Mail</h1><p class="empty">Mail is not available to you.</p></div>'; return; }
+  // Moving between messages (or back to the list) in the same folder changes only the reader and the highlighted row: the page is not rebuilt,
+  // so there is no flash, and the list keeps its place.
+  const shell = document.querySelector('.mailapp'), k0 = args[0], key0 = args[1];
+  if (shell && v.contains(shell) && mail.st && mail.st.connected && mail.accts.length && (k0 === 'm' || k0 === undefined) && (k0 !== 'm' || !String(key0).includes('~') || String(key0).split('~')[0] === mail.acct)) {
+    const view = $('#view');
+    if (k0 === 'm') { if (shell.dataset.mode !== 'read') mail.listScroll = view.scrollTop; shell.dataset.mode = 'read'; mailMarkSel(key0); mailOpen(key0); }
+    else { shell.dataset.mode = 'list'; mail.msg = null; mailDrawReader(); mailMarkSel(''); requestAnimationFrame(() => { if (mail.listScroll) view.scrollTop = mail.listScroll; }); }
+    return;
+  }
   // The accounts come from the server without any network call to the mail providers ("light"), so the page can draw at once; the folders, the message
   // list and the real unread counts then load side by side instead of one after another.
   let st = mail.st;
@@ -228,6 +237,7 @@ async function mailLoadList(reset) {
     mailDrawList();
   } catch (e) { if (showing) return; const el = $('#mrows'); if (el) el.innerHTML = `<p class="err" style="padding:12px">${esc(e.message)}</p>`; if (/Connect again/.test(e.message)) { mail.st = null; mcClear(); setTimeout(route, 1200); } }
 }
+function mailMarkSel(id) { document.querySelectorAll('a.mrow').forEach(a => a.classList.toggle('sel', !!id && decodeURIComponent(a.getAttribute('href').slice(9)) === id)); }
 function mailDrawList() {
   const rows = $('#mrows'); if (!rows) return;
   const sel = (location.hash.match(/^#\/mail\/m\/([^/]+)/) || [])[1];