thanks for the feedback. To the best of my knowledge it's not as easy as it sounds. We are already using a GDI capture for the "live" thumbnails of the RDP connections and one limitation here is that if hardware acceleration is enabled (DirectX rendering), it won't work. Capturing DirectX, OpenGL, etc. is much more difficult and there are no decent libraries out there to accomplish this task. In addition, capturing a window is only half of the story. The whole input handling (mouse/keyboard) is a challenge on its own. Maybe there's already something out there (open source or commercial) which solves all these problems but I'm not aware of any component which can easily be used from .NET.
So, reparenting seems to be the only feasible option right now, sorry. Should you be aware of any component which we can leverage, let me know. I'm happy to look into it.
Right now we have the option to add external applications via the external application option which launches said application within Royal TS.
Ultimately this can sometimes be both limiting and incompatible.
An alternative way of doings this could be via. a WinRT capture (or GDI as a slower compatibility option) which captures the window contents and then draws them within Royal TS.
While adding a program you would either select the window name from currently running windows, enter the window name yourself or even some variable functionality (as in - window name starts with or window name contains).
Basically how programs like OBS Studio select and capture windows.