When viewing your partner's screen, you can control the interaction using the toolbar at the top of the viewing window. If you fullscreen this window, you can get access back by moving your cursor to the top of the screen. Mouse and Keyboard mode settings will be persisted and automatically set as the default for your next call. The initial Stream Resolution setting is based on the host's preference but can be temporarily modified by the guest during a call.
Tag Team (default)
In Tag Team mode, one person has control of the mouse at a time. If you don't have control, you can take it by left-clicking once.
When a call begins, the person sharing their screen (the "host") starts with control.
If you are looking at someone else's machine (you are the "guest"), and you want to use the remote mouse for something, left-clicking once will take control. While the guest has control, the host's mouse movements won't affect the pointer.
Your clicks will create a pulsing indicator for a few seconds on the host's screen. Use this mode to quickly draw your partner's attention without getting too much in the way.
Just click and drag to draw on your partner's screen. Anything you draw will fade away automatically after a few seconds. You can also right-click to immediately clear all drawings.
Both you and the host have a cursor you can both see, and control switches behind the scenes. We have to do a lot of tricks to convince macOS there are two cursors (it doesn't want to believe this fact) so some interactions will be strange if both of you are trying to do things at the same time.
Your keystrokes will never be sent to the host machine. Use this mode if you will be switching back and forth between viewing your partner's screen and doing your own typing elsewhere, so you don't accidentally interfere.
This mode will send all your typical keystrokes through, but with the notable exception of Cmd+Tab and Cmd+Space. Use this mode if you will be doing some typing but prefer Cmd+Tab and Cmd+Space to execute on your machine. Keystrokes will be sent as long as the viewing window is focused.
Cmd Keys (default)
Full control of the host's machine. Use fullscreen for a truly immersive experience and become the driver of the remote machine. Keystrokes will be sent as long as the viewing window is focused.
Use this feature to directly send snippets of code or anything you want to your partner's clipboard. If you send something that looks like a web URL, it will open automatically in their browser (this can be disabled by the Host in Preferences).
If you aren't doing a lot of typing or clicking on your partner's machine, you should crank this up to get the sharpest image. However, if you want very snappy responses to your keystrokes, then you can get lower latency by reducing the resolution of the video.
If you experience interruptions in quality, it sometimes help to turn it off and turn it back on again :) This button lets you do that really quickly, automatically re-initiating the call with minimal disruption for the host. Note that audio will be lost for a brief moment.
This will take you straight to the documentation for the Guest Toolbar.
Tuple only allows fullscreen when the macOS setting 'Displays have separate spaces' is turned on. You can find that setting in System Preferences > Mission Control: