How do I seamlessly share a window from Mac OS X El Capitan to Windows 10?
I think that 'seamlessly share/remote access on window on my Mac' would benefit from some clarification about specific user scenario (is somebody at remote machine etc).
There is professional grade TeamViewer which is free for personal use which includes option to share whole screen or specific application window.
There is possibility to use Microsoft Teams to share from chat or meeting desktop, window or file.