Better IMAP: Messages labels prefetching

[kael]

A suggestion for fixing the lag when displaying labels on message load.

Instead of re-opening and closing an IMAP connection each time a message is loaded, how about using the IDLE command or the NOOP command every 29 minutes to maintain the connection opened ?

That way you'd need not to AUTHENTICATE/LIST/SELECT/FETCH each time a message is loaded.

Combined with a folder observer listening to folder switches, you can SELECT an IMAP mailbox upon folder changed (loaded/unloaded events, IIRC).

Also, with IDLE you can observe IMAP mailboxes and catch when a message label is edited. (Erratum: Actually X-GM-LABELS are not added in IDLE'd FETCH unsolicited responses, though)

At last, you could prefetch labels from messages visible in the mailview pane, and prefetch next ones each time the user scrolls.

[kael]

Assuming the IMAP connection would be persistent, labels could be fetched on demand using a custom column handler.

  
   // To be flushed on folder unselection
  ... this._messages = { labels: {}, awaiting: {} };

  nsIMsgCustomColumnHandler.getCellText: LabelOnDemand(uid) {
    if (uid in this._messages.labels) return this._messages.labels[uid];
    if (uid in this._messages.awaiting) return;
    const msgHdr = gDBView.getMsgHdrAt(uid);
    const labels = msgHdr.getStringProperty('X-GM-LABELS');
    if (labels) {
      this._messages.labels[uid] = labels;
      return labels;
    }
    const session = ...imapSession(msgHdr.folder.server);
    if (!session) return;
    this._messages.awaiting[uid] = true;
    FETCH(..., (response) => {
      const labels = response...;
      msgHdr.setStringProperty('X-GM-LABELS', labels);
      this._messages.labels[uid] = labels;
      delete this._messages.awaiting[uid];
      document.querySelector('#threadTree').invalidateRow(uid);
    });
  }

[postrational]

I don't mind the lag when fetching labels, but it makes all of Thunderbird unresponsive.