Classroom_Desktop/docs/classroom-v2-experiment.md

Classroom v2 architecture experiment

An open re-evaluation of the whole Ekklesia Classroom desktop stack, with reliability as the first requirement. Nothing here reaches a stable, canary or hyprland room, and nothing here changes the development host.

Base

branch	experiment/classroom-v2
base commit	aadb1b881977b0eb25e1ca10e060977bed31a48f
base subject	fix: keep the system print dialog in front of the window that rendered it
VERSION at base	0.1.44

That commit is the published stable build, verified rather than assumed: repo/stable/manifest.json names ekklesia-classroom_0.1.44_amd64.deb with sha256 7de568…81041, and the copy in repo/packages/ hashes to exactly that. The branch is not built on experiment/hyprland; git ls-files on it contains no Hyprland file.

The answer, in one paragraph

Every user-visible failure the church reported is a piece of desktop infrastructure the classroom wrote itself. The shell that disappeared had no supervisor. The minimised window with no feedback exists because wlroots has no minimise and the classroom invented one. The secondary window that fights the browser needed a 314-line arbiter because sway tiles. The unrecognisable browser icon was hand-drawn to match a monochrome set. Debian 13 already ships a desktop that solves all four, so the recommendation is Plasma 6.3 on the Debian the classroom already runs, configured rather than written, and it was built and tested here rather than argued for.

The development host, as found

Recorded because it bounds what could be tested.

OS	Debian 13 trixie (13.5), kernel 6.12.90
machine	already a VM — root is /dev/vda2, GPU is bochs-drm, CPU is QEMU Virtual CPU
nested virtualisation	none — /dev/kvm does not exist
libvirt / qemu / VirtualBox	not installed
containers	Docker 26.1.5, cgroup v2
resources	6 vCPU, 11 GiB RAM, 160 GiB free

This session could not create a virtual machine, so the candidate desktop was given a systemd container instead. Nothing outside Docker was changed on the host: no packages installed, no configuration touched, no service altered. See Alternate operating systems for what still needs a real VM.

The lab

tools/lab/ builds a Debian 13 container with a full Plasma 6.3 desktop in it, runs systemd as PID 1 so the session is supervised the way a real machine supervises it, and nests the session inside a headless sway so that grim can photograph it and a real pointer can be aimed at it.

tools/lab/lab build          # the image
tools/lab/cold-start 3       # three complete cold starts, from nothing
tools/lab/lab session        # compositor + Plasma session
tools/lab/lab shot NAME      # docs/screenshots/NAME.png
tools/lab/lab click X Y      # a real pointer press
tools/lab/kwin-script FILE   # ask KWin what it believes about a window

Every window measurement in this document came from kwin-script, not from looking at a screenshot. Every click came through libinput.

Three lab-only deviations, recorded because they bound what was proven:

• Software rendering. llvmpipe throughout, with KWin nested in sway. This proves behaviour and layout, not performance on classroom hardware.
• No sound card and no optical drive. The audio policy was therefore tested as a pure function against the real room's reported hardware state rather than end to end, and DVD playback was not exercised.
• Video did not decode. VLC came up fullscreen at exactly 1920×1080 with the panel covered, which is the window-management result being tested, but the picture was black under software XWayland.

What was measured

Each of these changed a decision.

The panel gives up after three crashes

Plasma's own unit restarts a crashed plasmashell, but with StartLimitBurst=3 inside sixty seconds. Measured:

kill 1	back in 0.3 s
kill 2	back in 0.4 s
kill 3	never came back

That is the production failure the church already had, reproduced on stock Plasma. A four-line systemd drop-in (systemd/user/plasma-plasmashell.service.d/ekklesia-classroom.conf) makes the restart unconditional. Re-measured: eight consecutive SIGKILLs one second apart, back every time in about 2.4 s, with the open application window untouched.

This is the single most important line in the whole experiment, and it is a supported override rather than a supervisor of our own.

KWin already recovers, and takes nothing with it

kill -9 kwin_wayland twice: back in 0.1 s each time, via kwin_wayland_wrapper, with the running application still on screen and its Wayland connection intact.

A deleted panel rebuilds itself

plasma-org.kde.plasma.desktop-appletsrc was deleted and the shell restarted. The classroom bar came back exactly as designed, from the layout script in the look-and-feel package. So the recovery procedure for a damaged desktop is one file removed and one service restarted — no reflash, and no technician in the room.

Plasma cannot show all five application states

The brief asks for closed, running, active, minimised and behind. Measured by sampling the pixels of the indicator strip rather than by eye:

state	indicator	icon
closed	none — panel background (26, 29, 32)	full colour
running, behind	grey (42, 45, 48)	full colour
running, active	teal (65, 127, 121)	full colour
minimised	grey (42, 45, 48) — identical to behind	full colour

Three marks for four states. Rather than write a task manager that draws a fourth — which is exactly the kind of custom desktop code this experiment exists to remove — the classroom removes the state: clicking a launcher always means show me this, never hide it, and the window decoration has no minimise button. There is nothing on the idle desktop a teacher could want to see, so minimising in this room only ever loses the lesson.

A window minimised by some other route is still shown as running and comes back with one click, so it can never be lost even then.

A window rule cannot tell a browser from its popup

The first attempt maximised the browser with a KWin rule on google-chrome. Measured: the window a lesson page opened was maximised too, because Chrome gives both the same class — the very behaviour the current architecture spends 314 lines preventing.

Chrome can tell them apart, so --start-maximized in the launcher does it. Measured afterwards:

main browser	[0,0] 1920×1004 — the whole desktop above the bar
window the page opened	[509,160] 902×683 — centred, the size the page asked for
main browser afterwards	unchanged

The secondary window carries an ordinary title bar with a close button, so the dock's sticky Close button has nothing left to do.

Printing

A page's own Print button produced a native KDE print dialog through xdg-desktop-portal-kde, transient=true on the browser, centred over it, in the classroom's colours, offering only Elmer. Stacking is the window manager's ordinary behaviour and needed no rule.

An immutable colour scheme cannot be toggled

Marking [General] immutable in /etc/xdg/kdeglobals froze the palette so hard that classroom-theme light changed the running session and lost the change at the next start. Immutability now covers the Kiosk restrictions and nothing else. Measured both ways afterwards.

VLC calls itself Vlc

KWin's window matching is case sensitive, and a lower-case substring rule matched nothing while reporting nothing. The rule is a regular expression now.

The architecture

distribution	Debian 13 trixie — unchanged, so the .deb and OTA work survives
desktop	KDE Plasma 6.3.6 (Debian's own packages)
compositor	KWin 6.3.6, Wayland
panel	Plasma panel, one full-width bar, built by a look-and-feel layout script
task feedback	Icons-only Task Manager
notifications	Plasma's own
browser	Google Chrome, unchanged
Wonder Ink	Chrome app mode, its own profile
session start	SDDM autologin (recommended; not yet tested — see risks)
lockdown	KDE Kiosk action restrictions
audio	one small classroom service, event driven

The bar

One continuous full-width bar, 76 px, the height the room already has.

• Left — Parent Call, drawn as a line mark in the accent rather than a colour tile, so it reads as a different kind of thing from the applications and is visibly not one of them.
• Centre — the applications: Wonder Ink, and the browser wearing Chrome's own icon, in colour. That is the direct answer to the teacher who asked whether the old monochrome glyph meant "the Internet".
• Right — network, volume, removable media, notifications, clock. Four applets placed directly on the bar rather than a system tray: the tray discovers whatever is installed and hides most of it behind an arrow, and in the lab it collected battery, brightness, clipboard, keyboard layout, camera, vaults and screen management on a machine with none of those.

The bar launches applications, shows their state, and shows a little status. It is not the Live View, not Parent Call, and not a shell framework.

Wonder Ink as an application

Wonder Ink   chrome --app=https://app.wonderink.org/ --user-data-dir=…/wonderink
Browser      chrome                                   the room's own profile

Its window class is chrome-app.wonderink.org__-Default — measured on a running session, and the launcher's StartupWMClass matches it exactly, so the panel shows one button rather than a launcher and a window side by side.

The separate profile buys something the current single-profile design cannot: if a teacher signs into Wonder Ink through Google, that Google session is in the Wonder Ink profile and closing the window ends it, while the church YouTube sign-in lives in the browser's profile and is untouched. The documented conflict in browser.md — that clearing wonderink.org leaves the next teacher one click from the previous teacher's account — does not arise.

The ordinary browser therefore no longer opens Wonder Ink. Its home page is YouTube, because a second way in through that window would put a teacher's lesson session in the room's shared profile, which is the leak the split exists to prevent.

Windows

KWin's ordinary behaviour, with one rule.

• Main windows fill the desktop above the bar, edge to edge, because Chrome is told to start maximized.
• Windows a page opens float over the main one, centred, at the size the page asked for, with a title bar and a close button.
• Dialogs are transient on their parent and stack correctly with no help.
• Fullscreen video covers the whole display: [0,0] 1920×1080, panel included, no rounding, no wallpaper.
• One rule, and it is noborder for VLC.

The outer margin and the 10 px rounding were dropped. Chrome squares its own corners when maximised, so margin and rounding are the same decision, and expressing "the screen minus 16 px" needs a rule generated per resolution at every boot. That is glue in exchange for appearance, and the brief says reliability wins. Full-bleed above the bar is also what every other appliance does.

Theme

One set of design tokens in EkklesiaDark.colors; the light scheme is generated from it by tools/build-light-scheme, so a colour can only be changed in one place. Plasma applies either to the panel, the notifications, the decorations and every dialog at once. classroom-theme light|dark|toggle is the whole switch. Accent teal #4FB3A8, taken from the wallpaper.

Wallpaper

config/wallpapers/classroom-calm.png is a stand-in, generated by tools/build-wallpaper in the sage/teal/blue/lavender palette with a river leading into the picture and a quiet upper middle. The illustrated wallpaper the church selected is not in this repository; when it arrives it replaces that file and nothing else changes.

Lockdown

/etc/xdg/kdeglobals, [KDE Action Restrictions][$i]. Verified by clicking, not by reading the file: right-clicking the wallpaper produced no menu, right-clicking the bar produced no menu, and KRunner was not running and could not be summoned. run_command, shell_access, logout, action/lock_screen, action/kwin_rmb and plasma/plasmashell/unlockedDesktop all read back false. There is no lock screen and no session restore.

Nothing was achieved by deleting a binary. Lifting a restriction for maintenance is one line and a session restart.

Audio

The one place custom code is still right, because no desktop expresses this policy. lib/classroom_audio.py is a pure function so it can be tested against the room that failed rather than against a machine:

• a port that reports itself unavailable is not a candidate;
• an analog port existing is not evidence that a cable is in it;
• an explicit AUDIO_OUTPUT=hdmi|analog overrides everything.

scripts/classroom-audio follows PipeWire's own event stream — no polling — and on every change sets the card profile, sets the default sink, and moves streams that are already playing. That last step is the fix for the teacher who had to restart the browser after changing the output.

Twelve test cases in tests/test-classroom-audio.sh, the real room among them. The service is supervised the same way the panel is; measured, it came back from four consecutive kills.

DVD

/usr/share/solid/actions/ekklesia-dvd.desktop with an X-KDE-Solid-Predicate for a video DVD. Plasma's device notifier offers "Play the DVD" while a disc is in the drive and withdraws it when the disc leaves, so a room without a drive shows nothing and no room needs DVD_ENABLED edited. Nothing polls, and classroom_optical's two-second loop goes away. Not exercised — the lab has no optical drive.

Custom code this deletes

	lines	replaced by
classroom-dock	754	the Plasma panel
classroom-chrome-windows	314	--start-maximized, and a window manager
classroom-window	208	the task manager
classroom-notify	214	Plasma notifications
classroom-notifications	74	—
classroom-scale	57	Plasma display scaling
classroom_sway.py	127	—
classroom_icons.py	95	the icon theme
classroom_style.py	36	the colour scheme
classroom_optical.py	125	Solid
sway, waybar, mako configs	—	—
config/bash_profile	2	SDDM
total	≈ 2 000

Kept, because they are the classroom's own subject matter: the DVD session and CSS setup, printer setup, provisioning, MQTT, the updater, shutdown. Added: the audio policy and its test, the theme switch, and configuration.

Alternatives, and why they lost

Stay on sway, add Waybar. Cheapest migration, and it loses on the thing that matters. wlroots has no minimise, so the invented hidden workspace stays; sway tiles, so the 314-line window arbiter stays; there is no window-manager decoration, so the dock's sticky Close button stays; and Waybar is one more unsupervised process to lose. It removes none of the four reported problems.

Hyprland (experiment/hyprland). Already measured on this repository and already carrying two findings against it: the legacy config silently matches nothing, and a Lua config changes the IPC language so the classroom would be adopting a brand-new format early. For critical production UI in a room nobody attends on a Sunday morning, that is the wrong end of the maturity curve.

GNOME. Task state, a bottom bar and application pinning all need extensions, and extensions break on GNOME updates. That is precisely the "several fragile independently-maintained components" the brief rules out.

Fedora KDE / Arch. Both would supply the same Plasma. Neither supplies anything Debian 13 does not, and both would cost the entire .deb and OTA investment. Arch's rolling updates are the wrong risk for an appliance that must survive unattended. Changing distribution buys nothing here, which is the most useful negative result in this document.

Firefox ESR / Chromium instead of Chrome. No concrete benefit. Firefox has no app mode, which is what makes Wonder Ink an application; Debian's Chromium ships no Widevine. Chrome is what Wonder Ink is known to work in.

Reliability results

test	result
cold start, 3 identical runs	16, 16, 17 s — panel, compositor and 11 applets every time
plasmashell SIGKILL ×8 at 1 s	back every time, ≈2.4 s
plasmashell on stock settings	dead on the third kill — fixed by the drop-in
kwin_wayland SIGKILL ×2	back in 0.1 s, windows survived
audio service SIGKILL ×4	active every time
panel configuration deleted	rebuilt itself from the layout script
right-click desktop / panel	no menu
KRunner	not running, not reachable
light ⇄ dark	persists both ways

Screenshots

In docs/screenshots/.

v2-idle-desktop the idle desktop · v2-light-bar / v2-dark-bar the same bar in both themes · state-1-closed closed · state-3-behind active beside running · state-4-minimized · state-5-restored · v2-testpage a maximised browser · v2-secondary-window a window a page opened, floating · v2-print-dialog · v2-notification-crop · v2-fullscreen · recover-panel the bar rebuilt from nothing · lock-desktop-rmb, lock-panel-rmb, lock-krunner the lockdown.

Not done, and honest about it

• DVD end to end. No drive in the lab.
• Real audio hardware. Policy tested as a function; no card to apply it to.
• Video decode and performance. Software rendering only.
• SDDM. Recommended over the current bash_profile exec, because when the session ends SDDM starts another one instead of leaving a teacher at a shell prompt. Not yet configured or tested.
• Packaging. tools/build-release.sh still installs the sway-era file list. The desktop is installed by tools/lab/install-desktop, which is the shape the package needs but is not the package.
• Parent Call and Live View. Placed in the architecture, not built. Live View's pin is KWin's Keep Above, which is already a title-bar button.
• UI scale. Plasma's own display scaling replaces classroom-scale; not exercised at 1.25 or 1.5.

Alternate operating systems

None is required. The recommendation stays on Debian 13, which the classroom already runs and already packages for.

If the church later wants Fedora KDE or Arch compared on real hardware, that needs a VM this session could not create:

Recommended next candidate: none. Debian 13 + Plasma 6.3 was actually tested, in a container, on this branch. A disposable VM is needed only for what a container cannot show: 4 vCPU, 8 GiB RAM, 60 GiB disk, a real GPU or working virtio-gpu, an HDMI-capable audio device, and an optical drive — to test DVD playback, audio switching against real hardware, SDDM autologin, display hotplug, and performance on classroom-class hardware.

The existing Debian classroom test VM should be kept.

Checkpoint log

• Branch opened at aadb1b8; host recorded as unable to create a VM.
• Lab built; Plasma 6.3.6 running software-rendered at 1920×1080.
• Panel supervision measured, failure reproduced, drop-in written, re-measured.
• Classroom desktop written: colours, layout, window rules, Kiosk, launchers.
• Task states measured; minimise removed as a route.
• Secondary windows, print dialog, notifications, fullscreen, themes, lockdown all verified on a running session.
• Audio policy written with the failing room as a test.
• Cold starts repeated.

Next: SDDM autologin; move the file list in tools/build-release.sh to the Plasma layout and build a 0.1.45~plasma1 package on its own channel; Plasma display scaling at 1.25 and 1.5; then a real VM for DVD, audio and performance.