Mac Mail message window position moves unexpectedly...

40 year Windows guy moved to Mac. Not yet comfortable with Mac Mail, but here is my issue:

Using three pane setup.

No matter how many times I open to compose a new mail (one at a time), it opens in the place and size I have chosen. Perfect.

BUT...I can open one email to read in the separate mail message window. (not preview pane). I close that and open a different email. For some number - maybe 3 to 6 or 8 times I do this, the message window opens just where I have set it. BUT then, on clicking that next message the message window will jump, most often to the other side of the screen! I manually move it back. Continue and this WILL repeat.

So, open a message anyway you like. CMD o or double click. Read the message. Close it anyway you like. Red Close dot. CMD W - whatever - even send to trash. Choose a different email message and repeat. After X number of time (might even be 1!) the window will change screen co-ordinates and most often dramatically. The position is NOT stepping, it is jumping across my monitor to a radically different position.

I even tried creating a shortcut to move the Window. And it works in other apps like Text Editor, but in Mac Mail cannot tell the difference between the message and the app so moves the message window AND the three paned app which might be a worse solution.

So the goal is to have EVERY message opening JUST one at a time in the same screen position.

Possible?

Does my setup have an issue and this actually works consistently for you?

Appreciate the help.

Thanks.

~Bob