Mail Window behavior (enlarging window)

[mmkerc]

First I am running an M1 Mini, with OS 11.6.5 (Big Sur). Recently when I (double) click on the window header (toolbar) the window enlarges to the width of the screen it is on (not full screen mode). I will have to manually resize back to the size I want. Does anyone know if this is a setting, or "new" feature in Mail? Most important is how to get it to stop?

 

[Toutou]

Preferences -> Dock & Menu Bar, uncheck or change "Double-click a window's title bar to ---"

It's not a new feature and it's not specific to Mail (try it in Finder or Safari).

 

[mmkerc]

thanks not sure how that got checked but it did stop it. Interesting side note it only worked in Mail. did not effect Safari, or other windows. - odd