feat: give the classroom a desktop instead of a shell

The whole appliance UI is now configuration for a desktop Debian already
ships: a Plasma 6.3 session with a colour scheme, a look and feel package
that builds the bar, window rules, and KDE's own Kiosk restrictions.

Two things were measured rather than assumed, and both changed a decision.

Plasma restarts a crashed panel three times a minute and then gives up,
which is the failure a classroom has already had. Four lines of systemd
drop-in make it unconditional; the panel then survived eight SIGKILLs a
second apart, back each time in 2.4s.

Plasma draws a minimised task and a background task identically, so the
five states the room needs cannot all be shown. Rather than write a task
manager, the classroom removes minimise: clicking a launcher always means
show me this.

Co-Authored-By: Claude Opus 5 <[email protected]>

tools/lab/lab

@@ -75,8 +75,19 @@ down)
     ;;
 
 outer-up)
+    # A headless compositor has no input devices, and a wl_seat with no
+    # pointer capability never gives the session under test a pointer to
+    # click with — sway's cursor commands then report success and move
+    # nothing. So the lab runs wlroots' libinput backend as well, over the
+    # devices the privileged container inherits, with seatd standing in for
+    # the login session it does not have. Clicks in this experiment are
+    # real pointer events arriving through libinput.
+    docker exec "$NAME" sh -c 'pgrep -x seatd >/dev/null || \
+        (SEATD_VTBOUND=0 setsid seatd -g input > /tmp/seatd.log 2>&1 &)'
+    docker exec "$NAME" usermod -aG input classroom 2>/dev/null || true
+    sleep 2
     session_env \
-        -e WLR_BACKENDS=headless -e WLR_LIBINPUT_NO_DEVICES=1 \
+        -e WLR_BACKENDS=headless,libinput -e LIBSEAT_BACKEND=seatd \
         -e WLR_RENDERER=pixman -e LIBGL_ALWAYS_SOFTWARE=1 \
         -d "$NAME" sh -c 'exec sway -c /srv/classroom/tools/lab/outer-sway.conf \
             > /tmp/outer-sway.log 2>&1'
@@ -91,6 +102,7 @@ plasma-up)
     session_env \
         -e WAYLAND_DISPLAY="$OUTER" -e XDG_SESSION_TYPE=wayland \
         -e XDG_CURRENT_DESKTOP=KDE -e LIBGL_ALWAYS_SOFTWARE=1 \
+        -e QT_LOGGING_RULES="kwin_scripting.debug=true" \
         -d "$NAME" sh -c 'exec startplasma-wayland > /tmp/plasma.log 2>&1'
     for _ in $(seq 1 60); do
         docker exec "$NAME" pgrep -x plasmashell >/dev/null 2>&1 && break
@@ -114,27 +126,45 @@ session)
     ;;
 
 shot)
-    # shot NAME [--delay SECONDS]
+    # shot NAME [GEOMETRY]  — geometry as grim wants it, e.g. "0,980 1920x100"
     name="${2:?usage: lab shot NAME}"
-    outer grim /tmp/lab-shot.png >/dev/null
+    if [[ -n "${3:-}" ]]; then
+        outer grim -g "$3" /tmp/lab-shot.png >/dev/null
+    else
+        outer grim /tmp/lab-shot.png >/dev/null
+    fi
     mkdir -p "$SHOTS"
     docker exec "$NAME" cat /tmp/lab-shot.png > "$SHOTS/$name.png"
     echo "$SHOTS/$name.png"
     ;;
 
 click)
-    # click X Y   — a real pointer press in the session under test
+    # Absolute, through sway's own seat. This only works because the seat
+    # has a pointer capability at all, which is what the uinput device in
+    # `outer-up` is for.
     outer swaymsg seat - cursor set "$2" "$3" >/dev/null
-    sleep 0.3
+    sleep 0.4
     outer swaymsg seat - cursor press button1 >/dev/null
-    sleep 0.1
+    sleep 0.15
     outer swaymsg seat - cursor release button1 >/dev/null
+    sleep 0.5
     ;;
 
 move)
     outer swaymsg seat - cursor set "$2" "$3" >/dev/null
     ;;
 
+drag)
+    outer swaymsg seat - cursor set "$2" "$3" >/dev/null
+    sleep 0.3
+    outer swaymsg seat - cursor press button1 >/dev/null
+    sleep 0.3
+    outer swaymsg seat - cursor set "$4" "$5" >/dev/null
+    sleep 0.3
+    outer swaymsg seat - cursor release button1 >/dev/null
+    sleep 0.4
+    ;;
+
 key)
     shift
     outer wtype "$@"
@@ -148,6 +178,8 @@ sh)
 run)      shift; session_env "$NAME" "$@" ;;
 irun)     shift; session_env -it "$NAME" "$@" ;;
 inner)    shift; inner "$@" ;;
+inner-d)  shift; session_env -d -e WAYLAND_DISPLAY=wayland-0 -e XDG_CURRENT_DESKTOP=KDE \
+              -e QT_QPA_PLATFORM=wayland "$NAME" "$@" ;;
 inner-it) shift; session_env -it -e WAYLAND_DISPLAY=wayland-0 -e XDG_CURRENT_DESKTOP=KDE "$NAME" "$@" ;;
 outer)    shift; outer "$@" ;;
 
@@ -164,9 +196,11 @@ usage: tools/lab/lab COMMAND
   plasma-down      stop the Plasma session
   shot NAME        capture the screen to docs/screenshots/NAME.png
   click X Y        press the pointer at a screen position
+  drag X1 Y1 X2 Y2 drag from one position to another
   move X Y         move the pointer
   key ARGS...      wtype, aimed at the session
   inner CMD...     run CMD as a client of the Plasma session
+  inner-d CMD...   the same, detached
   outer CMD...     run CMD against the lab compositor
   run CMD...       run CMD as the classroom user, no display
   sh               root shell in the lab