# Classroom display size (Plasma v2)

Each room keeps its own `UI_SCALE` in `/etc/ekklesia-classroom/device.conf`.
Package upgrades preserve the value. Plasma applies it at every classroom login.
Native display resolution is unchanged.

## Administrator setting

On the classroom keyboard press **Meta+Ctrl+Alt+Shift+D** to open Display size.
It is hidden from application lists and the teacher's panel. Saving requires
administrator authentication through polkit; the keyboard shortcut grants no
privilege. Choose a percentage and press Save and apply. Canceling the password
prompt leaves the saved setting unchanged.

Start at **200% for a 4K TV**, which gives 1920×1080 logical space while retaining
3840×2160 output. The offered values are 100, 125, 150, 175, 200, 225, 250 and 300%.
The choice applies to all connected, enabled displays on that classroom computer.

For SSH administration, the fixed-purpose helper can also be run as root:

```
sudo /usr/local/bin/classroom-set-scale 2
```

It writes only UI_SCALE, preserving room configuration, broker credentials and
file ownership and permissions. The new size takes effect at the next classroom
login, or by running `classroom-scale` inside the classroom's graphical session.
The hidden window saves and applies in one operation.

## Implementation and validation limits

`classroom-scale` uses `kscreen-doctor`; the login autostart runs only in KDE and
the script applies only to the classroom account. Values read from older/manual
configurations remain clamped to 0.8–3.0; invalid/nonfinite values become 1.0.
The helper accepts only the window's predefined choices, takes no paths, and
writes the root-owned settings file atomically. Polkit requires auth_admin for
every save, with no retained authorization.

Hardware checks required: shortcut registration, administrator authentication,
200% at 4K, logout/reboot persistence, and Chrome/Parent Call/DVD sizing. XWayland
applications may be softer than native Wayland applications when scaled.

## Wallpaper in the v2 package

The approved background source is `config/wallpapers/classroom-calm.png`. The
package installs it at `/usr/share/backgrounds/ekklesia/classroom-calm.png`, the
path used by the packaged Plasma layout. The layout is rebuilt at login, so
upgraded classrooms receive the package wallpaper without per-room copying.

## What the one number reaches

`classroom-ui-scale` prints it, clamped, and everything that cannot read
the display scale for itself asks that command rather than the file:

* **Plasma** — the panel, the clock, notifications, the volume display and
  every dialog, because `classroom-scale` sets it as the display's scale
  through `kscreen-doctor` at every login.
* **Chrome and Wonder Ink** — `--force-device-scale-factor`, because
  Chrome on Wayland was measured not to follow the compositor's output
  scale on its own. It is the same number, not a second scaling: Chrome
  replaces its detected scale with it.
* **The disc player's own surfaces** — they are measured in the logical
  pixels the compositor is already scaling, so they follow without being
  told.

The clock is the one thing that used to be left behind. It was pinned at
15 points, which is the same 15 points at every scale, and on a 4K
television it was a clock nobody at the back of the room could read. It
is sized by the bar now.