The actual handoff is precise: the window manager routes the event to the correct process based on its internal map. But the map is always slightly out of date. Because your input arrives asynchronously—interrupts, device drivers, kernel queues, compositor scheduling—the window manager must guess which window is actually under the cursor at the moment of click, not when the event was recorded.
The "actual window manager" is not a thing. It is a relationship—between hardware, kernel, compositor, and your hand on the mouse. And like all relationships, it works best when you stop analyzing it and simply trust the deception. I do not write this to make you fear your desktop. I write this because the window manager is the most used, least understood piece of software in your life. It mediates every click, every drag, every pixel of your digital work. actual window manager
But drawing is only the beginning. An actual window manager must enforce these borders. When you click at (x=400, y=200), the window manager must answer: which window owns this coordinate? Then it must route that click—not to the screen, not to the graphics driver, but to the specific process that owns that rectangle. The actual handoff is precise: the window manager
| System | What You Call It | What It Actually Is | |--------|------------------|----------------------| | Windows 11 | Desktop Window Manager (DWM) | A compositor + policy engine + input router, tightly coupled to the graphics kernel | | macOS | Quartz Compositor (part of WindowServer) | A userspace compositor + event manager + window database | | Linux (GNOME) | Mutter | A Wayland compositor + window manager + input manager | | Linux (KDE) | KWin | The same, but with pluggable window decoration and tiling scripts | | Linux (i3/sway) | i3 or Sway | A tiling window manager that is also a compositor (Sway) or relies on X11 (i3) | The "actual window manager" is not a thing
This is the first deception we accept: that we are clicking "on" a window. In reality, we are clicking on the window manager's map, and the window manager is whispering to each application, "Psst—someone touched your territory." To understand the gap between "window manager" and "actual window manager," we must discuss the compositor .