I struggled for a long time with the interface builder but I couldn't figure out how to create the constraints properly... @low-batt, maybe you can do that please?
Besides that, the PR seems to me to be ready.

I will take a look at this soon. Need to finish up two PRs I'm in the middle of at the moment.

I struggled for a long time with the interface builder but I couldn't figure out how to create the constraints properly...

@ShlomoCode I went ahead and did it. Take a look at bdb3c2a.

I couldn't figure out a way to submit an update to this PR directly, but feel free to copy & paste it into your code or whatever. I don't need attribution for it.

Forgot to add: to add logic to gray out the property when its "parent" (togglePipByMinimizingWindow) is unchecked:

@svobs Thank you! I just added a .patch at the end (https://github.com/iina/iina/commit/bdb3c2a84ed1b64e9309f39837be0c6a87df9df8.patch) and then saved to a file, run git apply and pushed.

I think this PR is ready

@uiryuu @CarterLi How can we continue with this?

Please be patient :) We are currently dealing with the remining issues for the first beta of 1.4.0 release. This PR will be included in the second beta.