A perfect example of breaking the “let me click” slash “the current action has the most momentum” principles we just talked about, in macOS Settings. When you try to add a new keyboard layout, the layouts you already have added are completely disabled, not just from addition, but even from selection:

On a surface this seems useful, right? A good signal to help you understand what you’ve already done? But this implementation also makes it unable to compare layouts of keyboards you have installed with the layouts you have not, since they previews are now in two different places. And this is important, as the differences between those layouts can be tiny, but significant.

This breaks momentum, and in the process also makes it hard to establish a good map of the entire space, since you are not allowed to walk around it freely. A tactical solution seems to be relatively standard at this point – disable the Add button instead, and indicate the already-added layouts in some other way.

Unrelated, but also – this popup-on-a-popup reminded me of this glorious old tram bezel city I’ve seen in Melbourne: