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In my experience, if you always hide the Mail window (cmd-h) instead of closing it (cmd-w), it won't ever pop back up. Takes a while to retrain the muscle memory though!

I haven't tested this in Catalina, but it definitely works consistently in Mojave.




This issue has been a real nuisance for at least 5 years and I could never retrain myself. One solution that worked for me was to make it full screen with its own Space.


Mind you, a program with “hidden” windows open takes up more CPU/memory than a program without any windows open, because the view-controllers for those windows are still running, just bound to an invisible, non-updating view. (Though, admittedly, it’s less of a CPU cost than minimizing the window, since that requires the view to be kept active too, to facilitate updating the live thumbnail.)

The main place this comes up is in cases where the main window is doing something clever to gather the information to display itself. For example, Activity Monitor doesn’t suddenly stop using half your CPU when you hide it.

Mail.app probably keeps a big scrollable list-control for the selected view in-memory for as long as you have the window open. So it might be a bad idea to do this if you’re one of those people who never archive anything (i.e. practice “Inbox Infinity.”)


> Though, admittedly, it’s less of a CPU cost than minimizing the window, since that requires the view to be kept active too, to facilitate updating the live thumbnail.

I thought that was a static thumbnail?

> For example, Activity Monitor doesn’t suddenly stop using half your CPU when you hide it.

It does for me, which is why I keep it hidden when I’m not actively using it.


> I thought that was a static thumbnail?

Nah, it updates (...as far as I can recall.) Try opening a chat client, minimizing the chat window, and then sending a message from another device to yourself.

I believe it’s just using the same call into the compositor that Mission Control uses to display your windows and spaces. Those are all live. (They might have a lower update rate, though.)

> It does for me, which is why I keep it hidden when I’m not actively using it.

Interesting. This might be down to Activity Monitor being written to respond to a message letting it know that its view is entirely obscured, and the Activity Monitor main-window view-controller deciding in response that there’s no point in it polling the system if all it’s going to do when re-visible is discard all the stuff it learned in the mean time and re-poll again to get the newest data for the view.

I can’t recall whether Activity Monitor has any historical/time-series views built in? If it does, then if you hide Activity Monitor with those active, it should keep using CPU, to gather the data for that view, whether it’s rendering it or not.


> Mind you, a program with “hidden” windows open takes up more CPU/memory than a program without any windows open

I can’t speak to Mail, but in general a Mac app with no visible windows may actually take zero CPU, because (if it’s written to allow for it) the OS may kill the process entirely.


Thanks a lot! I haven’t been bugged yet, but hope it works!! Would be a life saver when I need to watch full screen videos on VLC


This is what I've been doing to get "rid" of the error. Still on Mojave as well.




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