[Dalewyn]

It's the minimize/maximize/close buttons. Inactive windows draw them ever so slightly in gray compared to active windows drawing them in black. Calculator has them in black.

As for Powershell/Terminal, assuming the screenshot wasn't timed deliberately, there is no caret which implies it's not the active window.

It's shading, but the worst kind of shading.

[Numerlor]

The new windows GUI framework used for the windows on the screen seems to be worse in this regard, as normal windows get their titles greyed out which is quite obvious and is a more natural place to look at than the control buttons