GitLab

Following configuration in GUI

HTTP/HTTPS proxy
Address: example.com (https://example.com fails to parse)
Port: 443
Credentials: username:password

generates torrc with:

HTTPSProxy example.com:443
HTTPSAuthenticator username:password

When configured this way, Tor sends CONNECT request with my credentials over plaintext HTTP to port :443.

HTTPSProxy should be able to use HTTPS for proxying. HTTPS proxies are supported in Firefox, and could be configured via an extension, or a PAC file (which could be inlined in settings without using an actual file):

function FindProxyForURL(url, host) {
	if (host === "127.0.0.1")
		return "DIRECT";
	return "HTTPS example.com:443";
}

Trac:
Username: sf