komorebi/docs/dev/DESIGN.md

Design

This document explains why komorebi for KDE is built the way it is, one decision at a time. It is opinionated: these are the reasons behind the code, and reasonable people may disagree with them. Where a reason was not written down, the entry says that it needs one.

It grows with the code: a decision is added in the commit that makes it.

What is komorebi for KDE the port of?

komorebi has a Windows implementation, the original, and a macOS one. The Windows one is the reference. I used it as the reference for what to adapt, and I prefer what KDE and Wayland do natively over a workaround that the other two platforms need. The events the window manager reacts to follow the Windows ones (a window is shown, destroyed, focused, minimized, moved, resized), and a window is a small Copy handle, as there. The model the window manager tiles is the one both platforms have: monitors, each with workspaces, each with containers, each with windows.

Why is everything the window manager needs behind one trait?

So that the window manager can be tested without a compositor. A test that needs a real KWin has to boot a virtual machine, which takes a minute or more and needs QEMU with a hypervisor (vm-tests.md); a test against an in-memory desktop takes milliseconds and runs anywhere. The logic (which window goes where, what happens when one closes, how a stack cycles) is tested with the second kind, and the VM tests are kept for what only KWin can tell.

The cost is that a second implementation has to be kept honest. A fake that does what it is told is not KWin: a real client may refuse a size, a compositor may report an event the fake never raises, and a fake that is too permissive hides a bug that a compositor would show. The fake is therefore written to be strict where KWin is (a command about a window that no longer exists is an error) and to clamp where a real client does (a window with a minimum size keeps to it).

Why is the backend a process-wide singleton?

So that a window can be a small Copy value that carries only its id. A window that had to hold a reference to its backend would not be Copy, and every function that moves, focuses or hides one would take a context argument as well. That is a lot of plumbing for a thing that never changes while the daemon runs.

The cost is a global. A test that wants a private desktop overrides it for its own thread with with_backend, which restores the previous one afterwards, also on a panic.

Why does a backend answer from a table instead of asking the compositor?

A query that waits for the compositor can wait for a long time, and the window manager asks a lot of questions: every move of a container is followed by a look at where each window ended up. A backend keeps a table that the compositor's events update, so a query is a lookup. The trade-off is that the table can be a moment behind the compositor, which is why a command returns when it has been accepted and not when the client has acted, and why the frame a window really has is read back afterwards.

Why does komorebi not pick the next window to focus when one closes?

Because the compositor already has. When a window is destroyed, KWin activates the window it considers next and reports that as a focus change, which komorebi follows. If komorebi also focused a window of its own choosing, the two would fight over the focus. komorebi focuses the window of its focused container only when the compositor has left nothing focused.

Why does komorebi run a script inside KWin?

KWin has no protocol that lets another process read and set window geometry. Its scripting API has both, and a signal for each change komorebi follows: a window added, removed, activated, minimized, moved or resized. So the part of the backend that sees and touches windows is a script KWin runs, the bridge, and the daemon talks to it over the session bus.

Why does the bridge hold no logic?

So that everything the backend decides can be tested without KWin. A script runs only inside KWin: its own tests run it against a fake of KWin's API, which is only as right as the fake, and a test of it against KWin needs a virtual machine. The bridge reports what KWin says, as it says it, and carries out what it is told; what an event means (is this a window komorebi manages, has it been announced already, which id does it have) is decided by the table in kwin::state, which is plain Rust that takes the bridge's JSON and is unit-tested with it.

Why is the bridge type-checked and tested without a JavaScript build chain?

I wanted safety, tests and typing around the bridge that do not need a JavaScript build chain. KWin loads bridge.js as it is, so the checks read that file and produce nothing:

• tsc checks it as strict TypeScript through its JSDoc comments (checkJs, noEmit), against kwin-api.d.ts, a hand-written declaration of the part of KWin's scripting API the bridge uses. KWin publishes no declarations, and the declarations are narrower than the API where that catches a mistake: a property the bridge has no reason to set is read-only, and callDBus takes one string.
• bridge.test.js runs under node --test, node's own test runner, and loads the bridge into a node:vm context whose globals are a fake KWin.
• The messages of a session are JSON files that the bridge's tests and the Rust tests both read, so neither side can change a message alone.

tsc and node come from nixpkgs, through the dev shell and the flake's komorebi-bridge-js check. There is no package.json, no npm dependency and no bundler.

Why do commands go to the bridge as the answer to a held call?

Because it was the fastest channel measured, and it costs nothing while nothing happens. Four ways for the daemon to get commands to a script in KWin were measured in October 2026, on KWin 6.8.1 in a VM under HVF: twenty windows retiled in one batch, twenty times, timed from the send until every window reported its new frame.

Channel	p50 / p95 / max
a held Poll call, answered when there are commands	16 / 19 / 20 ms
the script polls on a 16 ms timer	16 / 20 / 32 ms
the script polls on a 50 ms timer	49 / 57 / 65 ms
a new script per batch (loadScript, run, unloadScript)	17 / 18 / 20 ms

A timer is as fast only at about 60 D-Bus calls a second, forever. A new script per batch is as fast, but 7 of its 17 ms (p50) are spent in the daemon's calls to KWin. A held call is answered as soon as there is a batch, and an idle bridge makes one call every 10 seconds. Rounds after idle gaps of 15 seconds to 10 minutes all completed, the slowest in 49 ms.

The hold is 10 seconds because QtDBus fails a call after 25 seconds. A call that fails is never answered (the callback of callDBus runs on success only), so the bridge has a one-second watchdog that calls Poll again when the last call is older than the hold plus 3 seconds, which is also how it finds a daemon that has restarted.

Why are events and the held call on two D-Bus objects?

Because events must arrive in order and a held call waits. zbus handles each method call in a task of its own by default, and two pairs of events were seen swapped. The Event interface is therefore spawn = false: zbus handles its calls inline, one after the other, in the order they arrive. The price is that a handler that waits holds up every call after it, so Event never waits, and Poll, which waits by design, is a separate object with the default spawning.

Why does everything cross D-Bus as a JSON string?

Because a script cannot choose the D-Bus type of what it sends. callDBus sends a JavaScript number as a 32-bit integer, and a call whose arguments do not match the method's signature fails with no error the script can see. One string argument always matches, and the daemon parses it. The declaration of callDBus the bridge is type-checked against takes one string, so tsc refuses anything else.

Why does the pointer follow the focus through a KWin shortcut?

Because a KWin script cannot move the pointer, and KWin can. Its "Move Mouse to Focus" action puts the pointer in the middle of the window that has the focus, and it can be invoked over D-Bus, through kglobalaccel: in the spikes it moved the pointer from the middle of the screen to the middle of the focused window, for two different windows. The backend asks for it once KWin reports the window it focused as active, so the action finds the right one. The cost is that the pointer can go to the middle of the focused window and nowhere else, which is what komorebi asks for.

Why does komorebi take every window out of its layout when KWin leaves the session bus?

Because nothing else will say that they have gone. When KWin goes away under a running session (it crashed, or another KWin replaced it), its clients lose their connection and their windows go with it, and so does the bridge, which would have reported them closing. In the spikes KWin was killed and started again under a running daemon on the same bus, twice: the old windows vanished without a single removed event. What the daemon does see is the bus itself: NameOwnerChanged says that org.kde.KWin has lost its owner, and later that it has a new one. So losing the name counts as every window closing, and getting it back as the moment to load the bridge into the new KWin, which then reports its desktop as it does at start.