fix(scene): browser sources paint on the GPU under X11 on Mesa

quest/m0/cef-headless.md

@@ -12,40 +12,53 @@ GPU path cannot paint fails its source saying why instead of staying
 [moq-dev/studio#59](https://github.com/moq-dev/studio/pull/59) picks the
 display at setup: Chromium's Wayland Ozone platform when `$WAYLAND_DISPLAY` is
 set, X11 when only `$DISPLAY` is, and the CPU path with neither. It passes the
-browser suite on the GPU path on NVIDIA (RTX 3070 Ti, GNOME on Wayland), and
-on the CPU path with no display on NVIDIA and Intel.
-
-Mesa did not paint in its only trial, on nixos2 (Intel UHD 630, Mesa 26.2.4,
-beside an RTX 2070), under a headless sway on the Intel render node, since no
-desktop session was logged in:
-
-- Wayland: CEF's GPU process starts on Mesa's EGL, but the first shared image
-  fails to allocate ("Unable to allocate frame for first frame capture:
-  OOM?") and the page never paints. The same happens with glvnd pinned to
-  Mesa (`__EGL_VENDOR_LIBRARY_FILENAMES`) and with `--disable-gpu-sandbox`.
-  Release CEF strips the allocation's own log (a `DLOG` in
-  `media/gfx/renderable_mappable_shared_image_video_frame_pool.cc`).
-- X11, under that sway's XWayland: shared textures import, but come back
-  empty (transparent pages, black WebGL).
-- The same headless sway on the NVIDIA render node paints shared textures, so
-  the harness itself works.
-
-First check whether a real Mesa desktop session paints; the headless sway on
-a hybrid host may be the cause. If it does not, find which allocation fails
-(a debug CEF build, or `chrome://gpu` in a CEF client on the same host).
-
-When the capturer cannot allocate, CEF never paints and never says so. Fail
-the page loudly instead, from a signal CEF or Chromium gives (no first-paint
-timeout).
+browser suite on the GPU path on NVIDIA (RTX 3070 Ti, GNOME on Wayland).
+
+Found on nixos2 (Intel UHD 630, Mesa 26.2.4, beside an RTX 2070):
+
+- X11 on Mesa paints since
+  [moq-dev/studio#73](https://github.com/moq-dev/studio/pull/73). ANGLE drew
+  through GLX there, which left every shared texture blank; the X11 path now
+  passes `--use-angle=gl-egl`.
+- Wayland is still unknown on Mesa. Under a headless sway on the Intel render
+  node, the first shared image fails to allocate ("Unable to allocate frame
+  for first frame capture: OOM?") and the page never paints. That headless
+  sway on the NVIDIA node paints, so the harness works.
+- GNOME with no monitor attached cannot run the Wayland path on any GPU. It
+  advertises no `wl_output`, and Chromium's `WaylandConnection::Initialize`
+  round-trips until one appears, so CEF's `initialize` never returns. The
+  helper hangs before reading a command and every page stays `Starting`.
+  Meanwhile `gnome-shell` uses a full core.
+
+Remaining:
+
+- Rerun `binary(browser)` on Wayland on the Intel GPU in GNOME with a display
+  connected, over ssh with `WAYLAND_DISPLAY=wayland-0`,
+  `XDG_RUNTIME_DIR=/run/user/1000`, and `VK_DRIVER_FILES` pointing at Mesa's
+  `intel_icd`. If it still fails to allocate, find which allocation fails (a
+  debug CEF build, or `chrome://gpu` in a CEF client on the same host).
+- Fail a page loudly when the GPU path cannot paint, from a signal CEF or
+  Chromium gives (no first-paint timeout):
+  - On allocation failure, Chromium's frame sink video capturer logs the OOM
+    line and calls `Stop()`, which calls the consumer's `OnStopped()`. CEF's
+    `CefVideoConsumerOSR` drops `OnStopped` and `OnLog` (empty bodies in
+    `libcef/browser/osr/video_consumer_osr.h`), so no client callback fires.
+    Surfacing it likely needs a CEF change upstream.
+  - When `$WAYLAND_DISPLAY` has no `wl_output`, decide that at setup rather
+    than hang in `initialize`: for example, check for an output in `raster()`
+    and fall to X11 or the CPU path.
 
 Decided 2026-11-06 (maintainer, /quest-unblock): this trial runs before
 nixos2 switches to Windows ([windows-host](/quest/m0/windows-host.md)), which
 then leaves no Mesa machine on NixOS.
 
-Since 2026-11-06 ([kixelated/nixos#31](https://github.com/kixelated/nixos/pull/31)),
-GDM logs `kixelated` into GNOME at boot on nixos2. Its GNOME Shell runs on
-the Intel GPU (`card1`, `renderD128`) with no monitor attached, so an agent
-runs `binary(browser)` there over ssh with `WAYLAND_DISPLAY=wayland-0` and
-`XDG_RUNTIME_DIR=/run/user/1000`. If a page paints only with a display
-connected, say so and ask the maintainer for an HDMI dummy plug on the
-motherboard's port.
+## Waiting
+
+- Maintainer: connect an HDMI dummy plug (or a monitor) to nixos2's
+  motherboard HDMI port, which the Intel GPU drives, since GNOME offers
+  Chromium no Wayland output without one. An agent cannot plug hardware, and
+  a virtual monitor would test a different setup from a real desktop. Check
+  that `cat /sys/class/drm/card1-HDMI-A-*/status` shows `connected` and that
+  `wayland-info` on `wayland-0` lists a `wl_output`. Alternative: skip the
+  Wayland trial on Mesa and let nixos2 switch to Windows, leaving the Wayland
+  result unknown. Recommended: the dummy plug.

quest/m0/cef-x11-nvidia.md

@@ -15,6 +15,11 @@ From [moq-dev/studio#59](https://github.com/moq-dev/studio/pull/59), picked
 - On nixos (RTX 3070 Ti, NVIDIA 580), the GPU path under X11 never paints:
   CEF's GPU process logs "Can't create buffer -- gbm device is missing".
   Wayland sessions avoid it since #59, but X11-only sessions still hit it.
+- Since [moq-dev/studio#73](https://github.com/moq-dev/studio/pull/73) the X11
+  path passes `--use-angle=gl-egl`, which made it paint on Mesa; rerun nixos
+  with it first. On nixos2 under GNOME's XWayland, with the studio on the RTX
+  2070, CEF still allocated on the Intel GPU that runs the X server (Intel
+  X-tiled modifier) despite `--render-node-override`.
 - First find why. Likely suspects: the NVIDIA GBM backend
   (`nvidia-drm_gbm.so`) is not on CEF's search path (`GBM_BACKENDS_PATH`), or
   `nvidia-drm` modeset is off. Fix it if the studio can, for example by

rs/moq-scene/src/browser/host.rs

@@ -496,6 +496,11 @@ wrap_app! {
 						Display::X11 => "x11",
 					};
 					command_line.append_switch_with_value(Some(&"ozone-platform".into()), Some(&platform.into()));
+					// On X11, ANGLE otherwise draws through GLX, which on Mesa
+					// leaves shared textures blank. Wayland is always EGL.
+					if *display == Display::X11 {
+						command_line.append_switch_with_value(Some(&"use-angle".into()), Some(&"gl-egl".into()));
+					}
 					// Paint on the studio's GPU, which is the only one it
 					// imports from.
 					if let Some(node) = render_node {