Since the JavaFX integrated browser does not work reliably for recaptcha and in general does not behave like standard browsers, I now use an external browser for the logins. The dependency to javafx-web has been removed.
The external browser is based on electron, which uses chromium as internal browser. The implementation can be found at https://github.com/0xboobface/ctbrec-minimal-browser
The browser is a minimal browser, which only shows the web page content without any other controls. It is launched by ctbrec in a new process and remote controlled over a socket connection. I first tried to control it via
stdin/stdout, but it turns out, that stdin is not supported by electron on windows.
Add synchronized modifier to the login methods, so that only one thread
at a time tries to login. All the following threads then should be able
to use the session cookies
0xboobface