Cannot snap windows to half screen on secondary display in macOS

Ahh the problem was the "Displays have Separate spaces" was turned off. Had to turn in on and the log out and then then back in and the the "Drag windows to left or right edge of screen to tile" works on both monitors

I cannot replicate the issue that you describe. These are Apple's instructions for tiling windows:

Tile windows on Mac - Apple Support

On my 16" M4 Pro MBP running macOS 27.2, those instructions work as stated on both the main and extended displays. In this case, the main display is an LG 5K:2K display and the internal display is secondary, but I tested with the opposite assignments and it works as expected. Also, I don't normally have the 'drag to left or right edge of screen to tile' option enabled, but I did so for this test.

Here is your post window on the secondary display:

If I click-and-hold the green stoplight button, I see the expected tiling options and they work normally:

If I drag the window to the left edge, I see the outline that you mention:

...and when I let the window go, it snaps to fill that half of the screen:

Since you are not seeing the behavior that is expected and documented by Apple, and at least two other experienced users cannot replicate the problem, it is likely to be an issue with your Mac.

Have you rebooted your Mac? If not, that's the first step. If that does not help, then see if the problem is present when you boot in safe mode. That process performs some system cleanup and clears caches, and also disables 3rd party extensions that may cause problems.

Start up your Mac in safe mode - Apple Support

If that does not correct the issue, test in a new user account. A newly-created user account will have all settings at default, and if the issue does not occur in that new account then it provides guidance on what may need to be changed to correct the issue.

Add a user or group on Mac - Apple Support

Once the testing is complete, the new account can easily be deleted.

Delete a user or group on Mac – Apple Support

Those steps will help to isolate the problem.

It might help to download and run EtreCheck, written by a long-time forum member. The free version is fine. Post the full report using the Additional Text button. No personal information is revealed. 

Good computing! Yes, when the Displays have separate spaces setting is off, then the external display doesn't really have an 'edge' as far as macOS is concerned.

Thank you for the clarification.

I had the setting for snapping to the side turned off, as I never used it - and I did not remember it.

I temporarily turned it on in order to test, and it seems to work in either side, or the corners, of either display - as soon as the mouse pointer reaches the border, a rectangular drawing of the expected position appears, and letting go of the mouse has the expected effect.

In summary, it appears that something is off in your mac.

Do you have any third party window manager running?

Try these two tests:

a) Start in Mac in Safe Mode, and see if things work correctly there

b) Start normally, and log in to a new user account. Do things work correctly?

How exactly are you trying to "snap" the window to half screen, and what happens when you try?

I tried it by clicking and holding the green dot, and also by using the commands in the Window menu, and it works on internal or external displays.

drag window to the side and it shows the outline of where it will fill and let go (like how windows has had for years). Works great on the main monitor

Holding the green dots for me does nothing other than stretch the little circle which is funny.

move left and right thru the window window works ok and I guess will be a temporary solution. Maybe this is a bug?

Also if I drag a window from the secondary monitor to the main one it not longer will snap either.

Ok I will give it a try.