wrapper: start mutter before chromium maps its window

Chrome on X11 negotiates client-side decorations (no WM titlebar) with the
window manager at window-map time. If mutter isn't registered yet, the
hint has no listener; when mutter later starts it reparents the existing
chromium window with default SSD — a WM titlebar with a close button
appears on top of chrome's own frame.

Move mutter's start to immediately after the X server comes up (was after
the dbus wait) and have chromium-launcher gate on mutter being registered
before exec'ing chromium (only when not --headless).

Author: sjmiller609

server/cmd/chromium-launcher/main.go

@@ -50,6 +50,17 @@ func main() {
 		fmt.Fprintf(os.Stderr, "warning: X display :1 not responsive after %s\n", d)
 	}
 
+	// Headful: wait for mutter to register before exec'ing chromium. If
+	// chromium maps its window with no WM present, the CSD hint it sends has
+	// no listener; mutter starts later, reparents the existing window, and
+	// applies default SSD — i.e., the titlebar with the close X. Headless
+	// has no WM, so skip.
+	if !*headless {
+		if d := x11.WaitForMutter(20 * time.Second); d >= 20*time.Second {
+			fmt.Fprintf(os.Stderr, "warning: mutter not registered after %s\n", d)
+		}
+	}
+
 	baseFlags := os.Getenv("CHROMIUM_FLAGS")
 	runtimeTokens, err := chromiumflags.ReadOptionalFlagFile(*runtimeFlagsPath)
 	if err != nil {

server/cmd/wrapper/main.go

@@ -151,12 +151,16 @@ func main() {
 	// (baked into the image at build time, see shared/envoy/bake-certs.sh)
 	// so it trusts the forward proxy on first start with no runtime cert
 	// work to wait on. chromium-launcher internally waits for the X server
-	// before exec'ing chromium, so we start it in parallel with the X
-	// server to overlap chromium-launcher's preamble with display startup.
-	// chromedriver listens on 9225 immediately and only attaches to
-	// chromium on session creation, so it can come up alongside everything.
-	// mutter has no internal X-wait, so it's started after the X server is
-	// confirmed up; neko is started with mutter on the WebRTC path.
+	// and (headful) for mutter before exec'ing chromium, so we start it in
+	// parallel with the X server to overlap chromium-launcher's preamble
+	// with display startup. chromedriver listens on 9225 immediately and
+	// only attaches to chromium on session creation, so it can come up
+	// alongside everything. mutter has no internal X-wait, so it's started
+	// as soon as the X server is confirmed up — chromium-launcher gates on
+	// it so chrome can negotiate CSD with the WM before mapping its window
+	// (without it, mutter reparents the existing window with default SSD
+	// and a titlebar appears). neko reads the active display mode at start,
+	// so it's deferred until after the dbus wait on the WebRTC path.
 	xServer := "xorg"
 	if prof == profileHeadless {
 		xServer = "xvfb"
@@ -171,13 +175,12 @@ func main() {
 	browserStart := time.Now()
 	startAll(xServer, "dbus", "chromedriver", "chromium")
 	waitForX(defaultDisplay, 20*time.Second)
-	waitForSocket(dbusSocket, 10*time.Second)
 	if prof == profileHeadful {
-		post := []string{"mutter"}
-		if webrtc {
-			post = append(post, "neko")
-		}
-		startAll(post...)
+		startAll("mutter")
+	}
+	waitForSocket(dbusSocket, 10*time.Second)
+	if prof == profileHeadful && webrtc {
+		startAll("neko")
 	}
 	browserDone := time.Now()
 

server/lib/x11/x11.go

@@ -3,6 +3,7 @@ package x11
 
 import (
 	"net"
+	"os/exec"
 	"strings"
 	"time"
 )
@@ -34,3 +35,23 @@ func WaitForDisplay(display string, timeout time.Duration) time.Duration {
 	}
 	return time.Since(start)
 }
+
+// WaitForMutter blocks until the mutter window manager has registered with
+// the X server, returning time spent waiting. Chromium negotiates CSD (no WM
+// titlebar) at window-map time; if mutter isn't up yet, it later reparents the
+// already-mapped window with default SSD decoration and a titlebar appears.
+// Polls `xdotool search --class mutter` to match the wrapper's readiness check.
+//
+// If the deadline elapses, WaitForMutter still returns; callers can compare
+// the returned duration against timeout to detect a miss.
+func WaitForMutter(timeout time.Duration) time.Duration {
+	start := time.Now()
+	deadline := start.Add(timeout)
+	for time.Now().Before(deadline) {
+		if exec.Command("xdotool", "search", "--class", "mutter").Run() == nil {
+			return time.Since(start)
+		}
+		time.Sleep(50 * time.Millisecond)
+	}
+	return time.Since(start)
+}