The next version of Steam Link for macOS is now available as a beta build.

You can opt into the beta on TestFlight by clicking on the link here:
https://testflight.apple.com/join/Cye7Gfyy

New in this build:

• Added a windowed mode option in the streaming settings
  
• Added the ability to specify exact values for streaming bitrate, framerate, and resolution in the streaming settings
  
• Streaming content is captured and scaled to match client window size
  
• Added a windowed mode toggle and 1:1 resize button to the mouse/keyboard overlay menu
  
• Window mode, position, and size are saved between sessions
  
• Stop streaming immediately when the close button is pressed
  
• Fixed a hang in streaming settings if the microphone isn't available