A rare bug in macOS' Screen Sharing service may result in the following symptoms:
- Before disconnecting a Screens session, mouse and keyboard input to the remote Mac may fail.
- Curtain Mode remains active on the remote Mac after disconnecting a Screens connection.
- Attempting subsequent connections to the remote computer results in a "Connection cancelled" message.
Apple is aware of and has acknowledged this issue, but no resolution has been offered as of yet. The Mac user community, however, has offered a few work-arounds you may try.
A potential solution could be to install Screens Connect 4.x on your Mac. When Screens Connect detects that a Screen Sharing connection has been closed, it will try to terminate the Curtain Mode process as a workaround for this issue.