vnc: only halve the stream when the framebuffer is actually large

-scale 0.5 was hardcoded on the assumption that :0 is 4K. With no monitor
plugged in X falls back to something tiny — 800x480 on this box — and halving
that served an unreadable 400x240.

Read the framebuffer width from xrandr and scale only above 2560px. When the
width cannot be read, serve 1:1: too many pixels beats a thumbnail.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/sidecar/vnc/vnc-manager.ts

@@ -10,6 +10,8 @@ const MIRROR_DISPLAY = ':0';
 const MIRROR_DISPLAY_NUM = 0;
 const MIRROR_PORT = 5900;
 const READY_TIMEOUT_MS = 5000;
+// Above this framebuffer width the stream is halved; at or below it, pixels are served 1:1.
+const SCALE_ABOVE_WIDTH = 2560;
 
 // x11vnc reads :0's cookie from the logged-in user's X authority, so no root is needed. Where that
 // cookie lives depends on the display manager: GDM (Ubuntu GNOME on Xorg) keeps it in the per-session
@@ -88,6 +90,20 @@ export async function ensureVncPassword(homeDir: string): Promise<{ password: st
   return { password, passwdFile };
 }
 
+// Width of :0's framebuffer, or null when it cannot be read (xrandr missing, X not up). Callers
+// treat null as "don't scale" — serving too many pixels beats serving an unreadable thumbnail.
+function getFramebufferWidth(xauthority: string): number | null {
+  const proc = Bun.spawnSync({
+    cmd: ['xrandr', '--current'],
+    env: { ...process.env, DISPLAY: MIRROR_DISPLAY, XAUTHORITY: xauthority },
+    stdout: 'pipe',
+    stderr: 'ignore',
+  });
+  if (proc.exitCode !== 0) return null;
+  const match = proc.stdout.toString().match(/current\s+(\d+)\s*x\s*(\d+)/);
+  return match ? Number(match[1]) : null;
+}
+
 async function isPortOpen(port: number): Promise<boolean> {
   try {
     const socket = await Bun.connect({
@@ -126,6 +142,16 @@ export async function startSession(params: VncStartParams): Promise<{ port: numb
     throw new Error(`No X authority found (GDM or ~/.Xauthority) — nobody is logged in on ${MIRROR_DISPLAY}`);
   }
 
+  // Halving a 4K framebuffer keeps the stream sane over the tailnet, but the same 0.5 applied to a
+  // small screen is just lost detail — and with no monitor plugged in, X falls back to something
+  // tiny (800x480 here), which halves to an unreadable 400x240. Scale only when there is genuinely
+  // too much to send.
+  const width = getFramebufferWidth(xauthority);
+  const scale = width !== null && width > SCALE_ABOVE_WIDTH ? ['-scale', '0.5'] : [];
+  if (width !== null) {
+    console.log(`[vnc] :0 framebuffer is ${width}px wide — ${scale.length ? 'scaling to 50%' : 'serving 1:1'}`);
+  }
+
   const proc = Bun.spawn({
     cmd: [
       'x11vnc',
@@ -140,9 +166,7 @@ export async function startSession(params: VncStartParams): Promise<{ port: numb
       '-localhost',
       '-forever',
       '-shared',
-      // :0 is 4K; halving the framebuffer keeps the stream sane over the tailnet
-      '-scale',
-      '0.5',
+      ...scale,
       '-noxdamage',
       '-quiet',
     ],