On the dev branch with RO, RSS and Principia, when launching to target LAN:

• starts the countdown and displays Autopilot status: Pre launch
• about 10 seconds before the launch, the autopilot button disengages, the countdown continues; pressing again the Engage autopilot button is blocked (it immediately reverts)
• once the countdown reaches 0, nothing happens, however pressing the Engage autopilot button at this point will actually launch the rocket

yeah that ones been around for months, i've just been lazy about fixing it...

should be fixed by #1856

After engaging the autopilot:

After pressing Launch to target LAN:

At about 10 seconds left to launch (immediately after the final countdown starts):