Mac Calendar appointment info window doesn't display with external monitor attached

I had the same issue.

A workaround is to go to Calendar > Settings > Advanced Settings

and then untick "Open events in separate windows".

This will display the "info" box on the calendar itself, avoiding the issues you mentioned.

Hello SF-Jeff,

Let's use the applicable set of steps below to boot your Mac into safe mode to see if that behavior persists from there:

Start up in safe mode

Find out whether you're using a Mac with Apple silicon, then follow the appropriate steps.

If you're using a Mac with Apple silicon

1. Shut down your Mac.

2. Press and hold the power button on your Mac. As you continue to hold the power button, your Mac turns on and loads startup options. When you see Options, release the power button.

3. Select your startup disk. It's named Macintosh HD, unless you changed its name.

4. Press and hold the Shift key, then click the “Continue in Safe Mode” button below your startup disk.

5. Log in to your Mac. You might be asked to log in again.

If you're using an Intel-based Mac

If you're not using a Mac with Apple silicon, you're using an Intel-based Mac.

1. Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.

2. Release the key when you see the login window, then log in to your Mac. 

3. You might be asked to log in again. On either the first or second login window, you should see ”Safe Boot” in the upper-right corner of the window. If you can't start up in safe mode, make sure that your Mac isn't using a firmware password.

You can also find those steps in this resource: Use safe mode on your Mac - Apple Support

It can take a few extra minutes to boot into safe mode. Your screen may flash or flicker while in safe mode. Those behaviors are expected. Testing in safe mode runs a check on your disk, removes some cached files, and can help isolate conflicts with startup items. Once you've tested that behavior in safe mode, you can restart to leave safe mode.

Additionally, does this occur every time you double click on a calendar event, or does this only occur intermittently?

Does that behavior return as soon as you reconnect your external monitor?

Let us know, and we'll go from there based on those details and your result in safe mode.

I have a similar issue with Calendar when connected to an external monitor, but only when the Calendar app is open and positioned on my external monitor. It happens every time I double click a calendar event or try to create a new event. The window to view or edit the event does not open. However, FYI, I have discovered that as soon as I close (i) move the calendar app to my MacBook Air, or (ii) when I close my MacBook Air, and therefore am only using my external monitor, in which case the 'details' windows does appear on the external window.

I have the same issue. M1 Max on 16" MacBook Pro. My work around is every single time I want to edit a calendar event, I have to quit calendar and restart it. Then I get the window(s) to appear.

MacOS 14.2.1

I have an external monitor. This drives me crazy too.

My issue was more that when I created a new event, or edited an existing event on my external monitor, the event window would pop up on my MacBook screen, in the upper left hand corner (literally as far away as possible!) I so I would constantly have to turn my head to look at it, and drag it over, then edit it as required. Your tip is a life and time saver! Thank you!! Having it open right on the calendar itself is PERFECT.