There should be options to capture single windows.
There is complication here, as different strategies may not work in even within a single OS.
IMO where possible capture should not capture the whole screen clipped to the window, where possible it should use native APIs to grab just the window.
On windows, one example of this is here
https://stackoverflow.com/questions/19695214/python-screenshot-of-inactive-window-printwindow-win32gui
It doesn't work for Modern UI apps, (so calculator does not work, but notepad does), where different APIs are needed, but it's probably better than nothing / a start ?
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