IIRC there are specific combinations of MediaStreamConstraints that affect the aspect ratio of the captured media. They should be documented in the developer documentation for chrome.tabCapture.

https://developer.chrome.com/extensions/tabCapture

It may also be useful to document the following:
- How tabCapture impacts fullscreen behavior
- That tabCapture runs even on background tabs
- That tabCapture causes a capture indicator to appear
- Only one tabCapture can run at a time