Screen/window sharing dialog should accept Esc key to cancel
When the screen- or window-sharing dialog is open on the screen, it can't be dismissed with the Esc key, a commonly-understood shortcut for "Cancel" in... well, basically any type of dialog or other transient window.
You can cancel the dialog with the keyboard, but you have to Tab to the Cancel button and hit Space or Enter. Esc should work when any part of the dialog is focused.