Based on checking the issue in testwiki wmf.6 and re-checking some scenarios from T108762: Notify users who are editing protected flow content the following issues are still present:

(1) (the issue described in this task) Cancel button is not functioning. A user can discard the editing window only by reloading the page.

(2) If the board description is collapsed, the Flow board doesn't have any indication that it's protected.

(3) The VE/source editor switch is active and a user can switch between two editors even though that the editing surface is not available.

(4) It's possible to open a permalink for a topic/reply. Since the board description is not present for permalink, there will be no indication that the page is protected.

(5) A user will get the message about the page being protected after attempting to publish their edits. The Cancel button works, but the unnecessary message about continuing or discarding the edits will be present.