komorebi/docs/dev/PLATFORM.md

Platform

komorebi for KDE runs in a KDE Plasma session, as a client of KWin. Most of the details on this page are also written down next to the code that depends on them; this page centralizes them: which KWin it needs, how it finds it, what it creates in the session and leaves behind, every environment variable it reads, and what happens when something is missing.

Facts that change with KWin and Plasma releases are dated. Unless a fact says otherwise, it was measured with KWin 6.8.1, Plasma 6.8.1 and Qt 6.12.0 from nixpkgs d3b13ad (2026-10-24), in NixOS virtual machines under QEMU on an Apple silicon Mac (HVF).

Vocabulary

• Session: one user's Plasma login: KWin, the Plasma shell, and a D-Bus session bus they share.
• Session bus: the D-Bus bus of the session. komorebi and KWin talk over it.
• KWin: the compositor and window manager of Plasma, kwin_wayland.
• Script: a JavaScript program that KWin runs inside itself, through its scripting API. komorebi's is the bridge, bridge.js.
• Output: KWin's name for a monitor.
• Headless KWin: kwin_wayland --virtual, a KWin with virtual outputs and no display, which the VM tests run (vm-tests.md).

What komorebi needs

KWin 6.8 or newer

KWin 6.8 is the oldest KWin komorebi supports. The bridge reads the work area of each output for the virtual desktop that output shows (workspace.currentDesktopForScreen), which is part of the per-output virtual desktops KWin 6.8 introduced. At time of writing (2026-10-27) KWin 6.8.1 is the only version komorebi has been run against; 6.7 and older have not been tried.

Wayland

komorebi runs in a Plasma Wayland session. There is no X11 session of Plasma 6.8 to run it in. Programs that run on XWayland inside the Wayland session are windows like any other; KWin reports their X11 class (XTerm) where a Wayland client has its app id (foot, org.kde.konsole).

The session bus

komorebi finds KWin on the session bus, and nowhere else. It needs to own the name org.komorebi.Bridge there, and KWin has to own org.kde.KWin. See DBUS_SESSION_BUS_ADDRESS for how the bus is found.

Environments

The same questions, for each place the daemon can be started:

1. How it finds KWin.
2. What it does there.
3. What it leaves behind.

A Plasma session

How komorebi finds KWin

Through the session bus. When the daemon is started from the session (a terminal, an autostart entry, a user unit of the session), the bus is already in its environment.

On NixOS, the module's user unit, komorebi.service, is wanted by plasma-workspace.target and ordered after plasma-core.target, which Plasma orders after KWin's service, plasma-kwin_wayland.service: the session starts the daemon once KWin's service has started (installation.md). The unit's PATH is /run/wrappers/bin, /etc/profiles/per-user/<user>/bin and /run/current-system/sw/bin.

What komorebi does

It connects to the session bus as org.komorebi.Bridge, writes the bridge to $XDG_RUNTIME_DIR/komorebi/bridge.js, and asks KWin to load it under the name komorebi-bridge, replacing a script of that name left by an earlier run. The script reports the windows and outputs it finds, and from then on every window that opens, closes, takes the focus, is minimized or brought back, moves or changes size. The daemon waits up to 10 seconds for the first report and does not start without it.

Every command of the window manager (move and resize, focus, raise, minimize, bring back or close a window) is queued for the bridge, which gets the queue as the answer to its held call and carries the commands out in KWin, in order. The script API cannot move the pointer, so when the pointer follows the focus komorebi invokes KWin's own "Move Mouse to Focus" shortcut, which puts the pointer in the middle of the window KWin has focused.

When KWin restarts

KWin can go away while the session goes on: it crashed, or kwin_wayland --replace took its place. The session bus stays, and the windows go with the old KWin. komorebi keeps running. It watches who owns org.kde.KWin on the session bus, and when nobody does it takes every window of the old KWin out of its layout, as if each had been closed: the bridge went with KWin, so nothing reports them closing. When the name has an owner again, komorebi loads the bridge into the new KWin, trying again for up to 15 seconds while KWin is not ready for it, and the bridge reports the new KWin's windows and outputs as it does at start. A window that opens in the new KWin is tiled. In the spikes the bridge was loaded again 1.0 to 1.1 seconds after the name had an owner again; the VM test vm-kwin-restart restarts KWin twice under a running daemon.

What komorebi leaves behind

When it is stopped with SIGINT, SIGTERM or a Stop message on its socket, it asks KWin to unload the script, closes an empty connection to each subscriber, and removes the socket. The file bridge.js stays in the runtime directory, which is removed at logout. A daemon that is killed leaves the script running in KWin, reporting to nobody, and the socket file in the runtime directory, until the next start replaces both or the session ends.

A headless KWin

kwin_wayland --virtual behaves like the KWin of a session for everything komorebi does: the VM tests run the daemon against one, on a private session bus of its own. It cannot draw (see Rendering), so nothing that depends on pixels can be tested against it. Started with --no-global-shortcuts, as the VM tests start it, KWin does not own org.kde.kglobalaccel, and the pointer does not follow the focus; the daemon logs that at debug level and carries on.

No KWin

Anywhere else (a session that is not Plasma, a Linux console, a Mac), the daemon exits with an error that names what is missing:

• "could not connect to the session bus as org.komorebi.Bridge": there is no session bus at the address the environment gives, or another komorebi owns the name.
• "KWin is not on the session bus": the bus is there and KWin is not.
• "KWin loaded the bridge script but it never reported in": KWin is there, and the script did not run. KWin logs a script's errors to the journal (journalctl --user -t kwin_wayland).

komorebi --fake-backend runs the window manager against an in-memory desktop instead, which works anywhere, for development and testing.

D-Bus

What komorebi owns, on the session bus:

Name	Object	Interface	Member	Called by
org.komorebi.Bridge	/org/komorebi/Bridge	org.komorebi.Bridge	Event(s)	the bridge, for every event
org.komorebi.Bridge	/org/komorebi/Bridge/Poll	org.komorebi.Bridge.Poll	Poll(s) -> s	the bridge, for commands

Both take a JSON string. Poll is held: the daemon answers it when it has commands for the bridge, or with an empty list after 10 seconds, and the bridge then calls it again.

What komorebi calls:

Name	Object	Interface	Member	When
org.kde.KWin	/Scripting	org.kde.kwin.Scripting	unloadScript(s), loadScript(ss) -> i	at start, and when KWin is back after a restart; unloadScript again at exit
org.kde.KWin	/Scripting/Script<id>	org.kde.kwin.Script	run()	after loadScript
org.freedesktop.DBus	/org/freedesktop/DBus	org.freedesktop.DBus	AddMatch(s), for the signal NameOwnerChanged	at start, to see KWin leave the bus and come back
org.kde.kglobalaccel	/component/kwin	org.kde.kglobalaccel.Component	invokeShortcut(s), with MoveMouseToFocus	when the pointer follows the focus

Files and directories

Path	What	Lifetime
$XDG_RUNTIME_DIR/komorebi/bridge.js	the script KWin loads, with its configuration (the D-Bus names and the hold time of Poll) on its first line	written at every start; the directory is removed at logout
$XDG_RUNTIME_DIR/komorebi/komorebi.sock	the command socket	bound once the daemon is connected to KWin, after a file left there is removed; removed when the daemon stops
$XDG_RUNTIME_DIR/komorebi/NAME	the socket of a subscriber, which the subscriber binds and names in its subscription	removed by the daemon, with the subscriber, when it cannot connect to it
$XDG_DATA_HOME/komorebi/komorebi.log.YYYY-MM-DD	the daemon's log, with colors, which komorebic log follows	a file a day (the UTC date), never removed by komorebi
$XDG_DATA_HOME/komorebi/komorebi_plaintext.log.YYYY-MM-DD	the same log without colors	as above

Without XDG_RUNTIME_DIR the bridge and the socket are in komorebi in the local data directory instead, with the logs (see XDG_DATA_HOME).

Environment variables

DBUS_SESSION_BUS_ADDRESS

The address of the session bus. If it is not set, komorebi uses $XDG_RUNTIME_DIR/bus, where systemd puts the session bus, and /run/user/<uid>/bus when XDG_RUNTIME_DIR is not set either. An address with no bus behind it is the first error under No KWin.

XDG_RUNTIME_DIR

The runtime directory of the user, normally /run/user/<uid>: komorebi writes the bridge to the komorebi directory in it, and binds its command socket there.

XDG_DATA_HOME

The logs are written to $XDG_DATA_HOME/komorebi (~/.local/share/komorebi if XDG_DATA_HOME is not set), which the daemon makes before anything else: a home it cannot write to stops it at once. When XDG_RUNTIME_DIR is not set, the bridge and the socket go there too.

RUST_LOG

Which log lines the daemon prints, in the syntax of tracing-subscriber's EnvFilter (komorebi=debug, komorebi::kwin=trace). If it is not set, --log-level sets it to komorebi=<level>; --log-level defaults to info.

RUST_LIB_BACKTRACE

Whether an error carries a backtrace. komorebi sets it to 1 when it is not set.

NO_COLOR

When set to anything but the empty string, the log the daemon prints on its standard output has no ANSI colors, which is what standard output redirected to a file wants. The VM tests set it. The log files are not affected: one has colors and the other none.

The bridge's environment

What the KWin scripting API offers a script, and what it does not, at time of writing (2026-10-27). The bridge is written around these, and kwin-api.d.ts declares the part of the API it uses, which tsc checks it against with ECMAScript 2016's library and nothing else. Node and TypeScript are needed only for those checks: in the session the bridge needs KWin alone.

• callDBus sends a JavaScript number as a 32-bit integer, and a call whose arguments do not match the method's signature fails without a word. The bridge sends every argument as a JSON string.
• The callback of callDBus is called on success only. A call that fails (the daemon has exited) is never answered, so the bridge re-issues Poll from a one-second QTimer when the last one is older than the hold time plus 3 seconds.
• There is a QTimer, but no setTimeout, and no Qt global: a rectangle is assigned as a plain {x, y, width, height} object.
• KWin activates a new window before it announces it (windowActivated comes before windowAdded). The bridge announces a window it has not seen when it is activated, so that no event names a window before the event that announces it.
• A script's print() goes to KWin's log, which shows it only when the kwin_scripting logging category is enabled (QT_LOGGING_RULES="kwin_scripting.debug=true", as the VM tests do).

How clients answer a frame

KWin applies a frame komorebi asks for at once, and then the client decides what size it draws at; the frame KWin reports afterwards (a geometry event) is the one the client chose. Measured at time of writing (2026-10-27) in a headless KWin with eight clients, by the VM test vm-client-geometry, and by the spikes before it with more sizes and at a scale of 1.25:

Client	Answer	In vm-client-geometry
Konsole, Kate, Dolphin (Qt, KDE)	exactly the frame asked for, also below the minimum size they report (spikes)	Kate asked for 948x527, has 948x527
xterm on XWayland	exactly the frame asked for	asked for 229x259, has 229x259
foot	sizes itself down to whole cells of its font	asked for 1905x2140, has 1902x2129
GTK 4 Text Editor	exactly the frame asked for, down to its minimum of 360x200	asked for 469x528, has 469x528
GTK 4 Calculator	never below its minimum of 360x616	asked for 469x259, has 469x616
Firefox	never below its minimum width of 500 and height of 120	asked for 230x259, has 500x259

Every client keeps the position it is asked for, also one partly off the screen. A fractional scale of 1.25 gives the same answers as 1.0: every whole logical frame comes back as it was asked for. komorebi does not ask again when a client keeps to a size of its own: the window manager reads the frame the window has from the table, and a window larger than its tile covers part of its neighbor's.

KWin moves a window anywhere it is asked to, off the screen too, but when it resizes a window it keeps 100 pixels of it on an output, and it does that once the client has drawn at the new size. A window that is moved off the screen and resized at once, as the scrolling layout does with the columns out of view, is pulled back onto the screen a moment later; so is one whose client rounds the size, as foot does. The bridge therefore remembers the position it was asked for, for a second, and when the window's frame changes to another position in that time, asks for the position again, which is a plain move. It tries three times at most, so a window that cannot go where it was asked is left alone. The VM test vm-scrolling checks that the columns out of view stay where komorebi put them.

How long a retile takes

Measured by the VM test vm-latency at time of writing (2026-10-27), in a headless KWin 6.8.1 under HVF on an Apple silicon Mac, with ten foot windows on a 3840x2160 output: twenty retiles (a window minimized and brought back, ten times over), each of which moved 5.5 windows on average. From the daemon's answer to Poll to the last frame KWin reported for the windows the batch moved, the client's answer included:

p50	p95	max
17.3 ms	27.7 ms	31.4 ms

The spikes measured the channel alone, with twenty windows in each batch, at 16, 19 and 20 ms (DESIGN.md).

Rendering in virtual machines and over RDP

At time of writing (2026-10-27):

• A headless KWin cannot render. Mesa's software rasterizer needs CREATE_DUMB on the primary DRM node and the virtual backend offers only the render node, so KWin reports that compositing is active while every frame fails, and screenshots are black. KWin does not start at all with no DRM device. The VM nodes give the guest a virtio-gpu device for that reason.
• In a QEMU guest everything is drawn in software (llvmpipe on the guest's CPUs): the nixpkgs QEMU has only the 2D virtio-gpu. KWin then refuses to load animating effects unless KWIN_EFFECTS_FORCE_ANIMATIONS=1 is in its environment, which the VM nodes set.
• KRDP 6.8.1, Plasma's RDP server, does not start without a TLS certificate and key, although its --help says it generates one; it logs the error and keeps running with nothing listening. xrdp is not an option, since there is no X11 session to start.