[Linux][Wayland] 3.50: app intermittently freezes at startup on Raspberry Pi (labwc, Mesa V3D); raster thread spins in eglMakeCurrent / wl_display_roundtrip_queue #196529

Steps to reproduce

1. Build a Flutter Linux app with Flutter 3.50.0 (stable) using flutter build linux --release on a Raspberry Pi (arm64). Our app plays a media_kit video right after startup.
2. Run it fullscreen in a native Wayland session: Raspberry Pi OS 13 (Trixie), labwc, Mesa 26.2 (gallium V3D).
3. Restart the app repeatedly.

Expected results

The app starts every time. It does when the same app is built with Flutter 3.47.8 on the same device.

Actual results

4 of 11 launches froze right after the first frame:

• The screen keeps showing that frame.
• The app's Dart code stops making progress: no more log output, and its local HTTP server stops answering.
• The process stays at ~55% CPU for as long as we waited (8+ minutes).

stderr repeatedly shows Timed out waiting for subsurface frame of size 1130x1970 (have 1080x766). This never appeared with the 3.47.8 build.

gdb -p <pid> -batch -ex "thread apply all bt", two samples 4 s apart, identical. The busy thread is io.flutter.raster:

#3  ppoll () libc.so.6
#5  wl_display_dispatch_queue_timeout () libwayland-client.so.0
#7  wl_display_roundtrip_queue () libwayland-client.so.0
#8  ??? libEGL_mesa.so.0
#16 driBindContext () libgallium-26.2.0
#19 ??? libEGL.so.1
#20-#29 ??? libflutter_linux_gtk.so

The main thread is in syscall().

Workaround

Building with Flutter 3.47.8, which doesn't use FlViewRendererSubsurface. In 3.50 the subsurface renderer is chosen for every Wayland display (fl_view.cc), so there is no opt-out other than GDK_BACKEND=x11.

Possibly related: #195822 (same renderer and same warning, on GNOME + NVIDIA).

Environment

• Flutter 3.50.0 stable, Linux arm64, Impeller OpenGLES
• Raspberry Pi, Raspberry Pi OS 13 (Trixie), labwc (Wayland), Mesa 26.2.0 (gallium V3D), GTK 3