When the window controls overlay is enabled for installed desktop web apps, the app's client area is extended to cover the entire window--including the title bar area--and the window control buttons (close, maximize/restore, minimize) are overlaid on top of the client area. The web app developer is responsible for drawing and input-handling for the entire window except for the window controls overlay.
Developers can use this feature to make their installed desktop web apps look like native app.
Docs: https://github.com/WICG/window-controls-overlay/blob/main/explainer.md
Samples: https://amandabaker.github.io/pwa/explainer-example/index.html
Explainers: https://github.com/WICG/window-controls-overlay/blob/master/explainer.md