I have a setup where I have three outputs: HDMI, integrated speakers and headphones. HDMI is in its own profile, while speakers and headphones share their profile. When speakers are available, headphones are not, and vice versa.

Let's say that the initial situation is that HDMI and speakers are available, so the dialog has two list items to select from, with HDMI selected initially:

    HDMI       (selected)
    Speakers

Then I select Speakers.

    HDMI
    Speakers   (selected)

Then I plug in headphones.

    HDMI
    Headphones (selected)

Then I plug out headphones.

    HDMI       (selected)
    Speakers

Wut? Why did HDMI get selected? My last action as a user (apart from plugging in and out the headphones) was selecting the speakers, so why did pulseaudio switch to HDMI when headphones were unplugged?

I confirmed that the request to use HDMI originated from gnome-control-center. My guess is that gnome-control-center noticed that there were some changes in the card state, queried the new state, and then iterated over the card ports. While iterating, the headphone port was processed first, and it had become unavailable. Since the currently active output was now unavailable, gnome-control-center decided to switch to the only available output, i.e. HDMI. After doing that, gnome-control-center continued the port iteration, and found that speakers had become available, so it added speakers to the output list.

If that speculation is correct, a possible fix would be to not tell pulseaudio to do any routing changes until the port iteration is finished.

I'll try to fix this, but I'm not confident that I'll get that done in any sensible time frame, so if someone else wants to try to fix this sooner, then awesome, go ahead.