We have seen this with a handful of clients -- typically with users of the new M1 MacBook Pro. In fact, I've seen it myself: however it was exhibited in OTHER programs, such as Microsoft Word and Apple's Xcode development environment.
The good news is we have a simple "fix" for when this happens (see below).
The bad news is it seems to be an Apple hardware (or possibly a MacOS software bug). Bad because despite our attempts to get Apple to confront the problem, including providing evidence and logs from clients' machine, all Apple could tell us is that the problem was NOT occurring inside Movie Magic Screenwriter.
Here's what is happening:
It appears that something is locking up MacOS from receiving typing and mouse clicking events. Typing or clicking can appear to freeze up for typically 10 seconds, but as long as 30 seconds. By that point, you've been moving the mouse around and typing, perhaps accidentally selecting large sections of text, but since no drawing / updating is taking place you can't see what is being selected, and the moment you type on the keyboard you replace/erase the selected text!
Once you realize things are messed up or frozen, how do you regain control of your mouse and keyboard events?
Do NOT attempt to quit or even "force quit" the Screenwriter application! If you do, you won't be able to undo and recover once you regain control of your mouse and typing.
There are two important aspects to recovering:
FIRST: We have discovered a QUICK method to fix this problem that even Apple was unaware of: in Apple's System Preferences / Options, select the "Desktop & Screen Saver" options. There should be a "Hot Corners" button, which lets you set a corner of the screen to start the screen saver. When your application (like Screenwriter) isn't responding, just move your mouse into that corner of the screen, which will start the screen saver. You can immediately move your mouse back (using touch ID to log in if needed), and you will now have unfrozen control. Then if you click, you'll see all your mouse and keyboard events play out.
SECOND: Now, since things might have gotten messed up, you're in luck: Screenwriter's Perfect Forward Undo will let you walk backwards through the undo states, no matter what has happened. It might not look correct on the first few undo operations, but keep doing CMD-Z, and eventually you WILL get back to where you were working. If you undo too far back, you can always redo forward.
This problem seems rare, and unrelated to Screenwriter. We have seen this rare behavior on Catalina, Big Sur, and Monterey. Hopefully, Apple will correct the problem in future versions of MacOS.