This issue applies to all Mac users who are running macOS Catalina (10.15) and are using the following browsers to join meetings:
Google Chrome (version 77 or below)
After selecting a screen/window to share, the "Share Screen" button normally changes to "Stop Sharing" and other participants can see the presentation. Due to this issue, the button continues to be labelled "Share Screen" and the screen sharing does not actually start. Other participants on the call will not be able to see the presentation.
This issue occurs even after allowing your web browser to record your screen in your Mac's Security & Privacy preferences, a requirement for screensharing. All of Highfive's Security & Privacy requirements can be found here.
This issue is caused by a web browser bug that any WebRTC-based video conferencing service will encounter when trying to screen share. (Highfive is not the only service impacted by this issue.)