Details like these demonstrate just how terrible many design decisions in @LiquidGlass really are.
To complement Mario's screenshots below, here's what the top left corner looks like when the sidebar in Mail.app is collapsed. Like Mario writes, that toggle button is now in the overflow menu on the other side of the menu bar. https://mastodon.social/@marioguzman/116003038206224260
Also, placing the button trio of Close/Minimize/Fullsize inside the sidebar outline creates the perception that those control are related to the other controls inside the sidebar. Which is obviously not the case, as they affect the entire window.
When the side bar is hidden, though, those three buttons now properly appear outside any outlined area, and are properly grouped with the window's title bar.
That's what you get when you ignore what you’d learn in #IxD 101. ☹️
Jochen Wolters