The window manager is the cartographer of this empty territory. It draws lines where none exist, declaring: "From pixel 320 to pixel 960, this region belongs to Firefox. From pixel 0 to pixel 320, this region belongs to your terminal."
Thus, the "actual window manager" is not a neutral piece of infrastructure. It is an opinionated choreographer of human attention. If we dig deeper than the compositor, past the input router, beyond the policy engine, we find the kernel and the display server .
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.
On Linux (Wayland), the kernel's DRM (Direct Rendering Manager) and KMS (Kernel Mode Setting) control the actual display hardware. The compositor talks to DRM via libdrm to flip buffers. On Windows, the DWM talks to the DXGI kernel driver. On macOS, WindowServer talks to the IOKit framebuffer.