I have no idea what stage manager is and I don’t wanna look it up. And I have an iPad. But it’s also running the older ios
It’s just yet another way to manage windows. It’s fully optional, I never even remember it exists. It was introduced in iPadOS 16 / macOS 13.
Fortunately, I found that there is now a way to reduce the stupid corner radius ("defaults write -g NSConvolutionOverride1 -float 8") and to remove the floating sidebars ("defaults write -g NSSplitViewItemSidebarDefaultsToFloatingAppearance -bool false").