Connectivity status is always displayed as "Connecting" when the server does not support IDLE

[link2xt]

When the server does not support IDLE, core takes the following path at the end of synchronizing seen flags:

deltachat-core-rust/src/scheduler.rs

Lines 563 to 575 in a119b24

	connection.connectivity.set_connected(ctx).await;
	ctx.emit_event(EventType::ImapInboxIdle);
	if let Some(session) = connection.session.take() {
	if !session.can_idle() {
	info!(
	ctx,
	"IMAP session does not support IDLE, going to fake idle."
	);
	return connection
	.fake_idle(ctx, Some(watch_folder), folder_meaning)
	.await;
	}

Connectivity is set to "Connected", then we go into fake_idle.

Normal idle does not change connectivity, while fake_idle calls prepare here:

deltachat-core-rust/src/imap/idle.rs

Lines 158 to 164 in a119b24

	// try to connect with proper login params
	// (setup_handle_if_needed might not know about them if we
	// never successfully connected)
	if let Err(err) = self.prepare(context).await {
	warn!(context, "fake_idle: could not connect: {}", err);
	continue;
	}

prepare calls connect which in turn changes the connectivity to connecting:

deltachat-core-rust/src/imap.rs

Line 305 in a119b24

self.connectivity.set_connecting(context).await;

However, nobody sets the connectivity to connected for the whole fake idle duration, so most of the time "connecting" is displayed.

For a fix it should be sufficient to set the status to connected at the end of connect. For testing we probably need an option to disable use of IDLE and maybe even add it to developer options on desktop.

163.com does not support IDLE.
Forum thread: https://support.delta.chat/t/cannot-connect-to-the-inbox-after-logging-into-163-mailbox/2756

[link2xt]

As a first step, adding an option to disable IDLE manually: #4803
Corresponding desktop PR: deltachat/deltachat-desktop#3427

Actual fix: #4804