ci: add headless smoke + screenshot test for the bar

Launch the real Waybar binary inside a headless, software-rendered sway
compositor and verify it actually runs and renders — something the unit
tests and build jobs never do.

- test/smoke/run.sh: boots sway (WLR_BACKENDS=headless, pixman), starts
  waybar, asserts it stays alive with no fatal log, optionally grabs a
  screenshot with grim
- level 1: real modules (clock/cpu/memory/disk) load without crashing
- level 2: deterministic config is screenshotted and checked to be non-blank
- level 3: screenshot compared to test/smoke/reference.png (fuzz 8%,
  800px tolerance); screenshot + diff uploaded as artifacts
- .github/workflows/smoke.yml runs it on push/PR

The reference image must be blessed from a CI artifact (see test/smoke/README.md).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/smoke/README.md

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+# Smoke / screenshot test
+
+The [`smoke`](../../.github/workflows/smoke.yml) workflow launches the real
+Waybar binary inside a **headless sway** compositor (software-rendered, no GPU)
+and checks three things:
+
+1. **It starts and stays alive** — with real modules (`clock`, `cpu`, `memory`,
+   `disk`) loaded, Waybar must not crash or log a fatal error.
+   ([`modules-config.jsonc`](modules-config.jsonc))
+2. **It renders** — a deterministic config ([`config.jsonc`](config.jsonc) +
+   [`style.css`](style.css)) is captured with `grim`; the bar strip must not be
+   blank.
+3. **It looks right** — the screenshot is compared to `reference.png` with an 8%
+   fuzz and an 800-pixel tolerance. Regressions fail the job, and the screenshot
+   and diff are uploaded as artifacts.
+
+## Updating the reference image
+
+`reference.png` must be generated by the CI environment (local rendering differs
+from Ubuntu's). To (re)bless it:
+
+1. Push your change and open the run of the `smoke` workflow.
+2. Download the `waybar-screenshot` artifact.
+3. Commit its `bar.png` as `test/smoke/reference.png`.
+
+Do this whenever an **intended** visual change makes the golden comparison fail.
+
+## Run locally
+
+On a machine with `sway`, `grim` and `waybar`:
+
+```sh
+./test/smoke/run.sh test/smoke/config.jsonc test/smoke/style.css /tmp/bar.png
+```

test/smoke/config.jsonc

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+// Deterministic Waybar config for the CI smoke/screenshot test.
+// Uses only static custom modules (interval "once") so the rendered bar is
+// reproducible and can be compared against a reference image.
+{
+    "layer": "top",
+    "position": "top",
+    "height": 30,
+    "modules-left": ["custom/left"],
+    "modules-center": ["custom/center"],
+    "modules-right": ["custom/right"],
+    "custom/left": {
+        "format": "Waybar",
+        "exec": "echo",
+        "interval": "once"
+    },
+    "custom/center": {
+        "format": "CI smoke test",
+        "exec": "echo",
+        "interval": "once"
+    },
+    "custom/right": {
+        "format": "12:34",
+        "exec": "echo",
+        "interval": "once"
+    }
+}

test/smoke/modules-config.jsonc

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+// Waybar config for the CI smoke test: exercises real modules that work in a
+// headless environment (no hardware/daemon dependency). Its content is dynamic
+// (time, cpu, ...) so it is NOT screenshot-compared — it only checks that these
+// modules load and render without crashing the bar.
+{
+    "layer": "top",
+    "position": "top",
+    "height": 30,
+    "modules-center": ["clock", "cpu", "memory", "disk", "custom/hello"],
+    "clock": {},
+    "cpu": {},
+    "memory": {},
+    "disk": {},
+    "custom/hello": {
+        "format": "ok",
+        "exec": "echo",
+        "interval": "once"
+    }
+}

test/smoke/run.sh

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+#!/usr/bin/env bash
+# Launch Waybar inside a headless sway compositor and verify it starts and
+# renders. Used by the `smoke` CI workflow.
+#
+# Usage: run.sh <config> [style] [screenshot.png]
+#   - always: assert waybar launches, stays alive, logs no fatal error
+#   - if <screenshot.png> given: capture the bar with grim
+#
+# Requires: sway, grim, waybar (on PATH). Software-rendered, no GPU needed.
+set -euo pipefail
+
+CONFIG="$1"
+STYLE="${2:-}"
+SHOT="${3:-}"
+
+export XDG_RUNTIME_DIR="$(mktemp -d)"
+chmod 700 "$XDG_RUNTIME_DIR"
+export WLR_BACKENDS=headless
+export WLR_RENDERER=pixman
+export WLR_LIBINPUT_NO_DEVICES=1
+export LIBGL_ALWAYS_SOFTWARE=1
+
+LOG="$(mktemp)"
+SWAYCFG="$(mktemp)"
+
+wb="waybar -c $CONFIG"
+[ -n "$STYLE" ] && wb="$wb -s $STYLE"
+
+cat > "$SWAYCFG" <<EOF
+output HEADLESS-1 resolution 1920x1080 background #000000 solid_color
+exec "$wb -l debug > $LOG 2>&1"
+EOF
+
+echo "::group::Starting sway (headless) + waybar"
+sway -c "$SWAYCFG" &
+SWAY_PID=$!
+
+cleanup() {
+    swaymsg -q exit 2>/dev/null || kill "$SWAY_PID" 2>/dev/null || true
+}
+trap cleanup EXIT
+
+# Wait for the Wayland socket to appear.
+sock=""
+for _ in $(seq 1 40); do
+    sock="$(find "$XDG_RUNTIME_DIR" -maxdepth 1 -name 'wayland-*' ! -name '*.lock' 2>/dev/null | head -1 || true)"
+    [ -n "$sock" ] && break
+    sleep 0.5
+done
+[ -n "$sock" ] || { echo "::error::compositor did not create a Wayland socket"; exit 1; }
+export WAYLAND_DISPLAY="$(basename "$sock")"
+echo "compositor up on WAYLAND_DISPLAY=$WAYLAND_DISPLAY"
+echo "::endgroup::"
+
+# Give modules time to poll/settle.
+sleep 6
+
+echo "::group::Waybar log"
+cat "$LOG" || true
+echo "::endgroup::"
+
+# --- Level 1: smoke ---
+if ! pgrep -x waybar >/dev/null; then
+    echo "::error::waybar is not running — it crashed on startup (config: $CONFIG)"
+    exit 1
+fi
+if grep -iqE 'critical|segfault|\bfatal\b|terminate called' "$LOG"; then
+    echo "::error::waybar logged a fatal error (config: $CONFIG)"
+    exit 1
+fi
+echo "✓ waybar launched and stayed alive under headless sway"
+
+# --- Level 2: screenshot ---
+if [ -n "$SHOT" ]; then
+    grim "$SHOT"
+    echo "✓ captured screenshot -> $SHOT"
+fi

test/smoke/style.css

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+/* Deterministic stylesheet for the CI smoke/screenshot test.
+ * A pinned font (DejaVu Sans, installed in CI) and fixed colors keep the
+ * rendered bar reproducible across runs for the golden comparison. */
+* {
+    font-family: "DejaVu Sans";
+    font-size: 14px;
+    border: none;
+    border-radius: 0;
+    min-height: 0;
+}
+
+window#waybar {
+    background-color: #1e1e2e;
+    color: #cdd6f4;
+}
+
+#custom-left,
+#custom-center,
+#custom-right {
+    padding: 0 16px;
+    color: #cdd6f4;
+}
+
+#custom-left {
+    font-weight: bold;
+    color: #89b4fa;
+}