It’s not, unless I’ve missed something. You can capture an image of a single window (direct from the compositor), but not video.
What you can do is select a region of a screen to video capture with cmd+shift+5. But that’s capturing from the framebuffer, so it captures whatever’s below the rounded corners as well (and doesn’t capture the window shadow).
QuickTime can record a window... low computational overhead, and built-in.
Can it? File / New screen recording just gets me the same cmd+shift+5 interface.
"capture selected window" or just press space
But that’s just a screenshot. You can’t do that with video right?
With both.. it's the same interface
You can't record _videos_ of a specific window though. You can only record a fixed region of the screen or the whole screen. "Capture selected window" only deals with screenshots, not with video.
Also when you move or resize the window?
Or can you just set the fixed frame for screen recording to a selected window's initial frame?
QuickTime + iMovie gets you what you need for anything short of full-on professional videos.