Thanks for tipps!
Accidently i figured out another method wich works very good, or better, it works perfect!
I do Screencapture whole screen, here a save-Dialog popup (in my configuration),
i do leftclick trayicon so main window appear, the power relies under the hood, and this one is mighty!
In my case it already has pre-selected my target window,
i click on crop to selection,
Sorry for asking i should have tried more first. (before i tried all combinations with grab window/grab region, no success)
And if you need an idea for a new feature, i hope that isnt included and i just did not found it, add something like a drag-crosshair that can be moved by mouse. Idea would be a feature that highlight any found [Control].[Handles]. I am bad at explaining but i hope it is understandable.
For example, dragging into a TotalCommander window would enable to grab region of control under mouse, like left or right files view.