Microsoft is working on a new toggle which is meant to make it easier for users to disable the Windows 11-styled rounded corners for browser windows in Edge. Microsoft already allows disabling and enabling rounded corners for browser windows inside Settings. The option dubbed "Use rounded corners for browser windows" is available in the Appearance section of Edge Canary. This can be toggled on or off.
However, with the new Canary, Microsoft is apparently experimenting with a new handy option to toggle on or off browser windows right on the three-dot menu. This means users will not have to navigate all the way to the three-dot menu > Settings > Appearance. The change was spotted by Deskmodder and you can see how the option would work in the images below. The left image shows the new toggle which is present right at the top while the right image shows the change after the browser was restarted with the toggle set to disabled.
Notably, this option is not available for everyone as it seems to be a controlled feature rollout. This feature could be available to all once it is ready for the Stable channel release sometime in the future.