saphid--frame-control/docs/webxr-chromium.md

WebXR in Chromium on the Frame

Goal: open a web VR180 or 360 player (DeoVR and DL8 embeds, WebXR samples), press its VR button, and watch in 3D in the headset.

The build and installer now live in their own public repo, saphid/chromium-webxr-steam-frame: a build script for an x86-64 Linux host, the SO_PEERCRED patch, and a Frame-side installer that adds "Chromium XR" to the Steam library. This page keeps the findings and what was verified on this Frame.

Why Flathub Chromium can't

Verified 2026-09-25 (Frame BUILD_ID 20260922.6101926, Flathub org.chromium.Chromium 154.0.8037.57 aarch64):

• navigator.xr exists, but isSessionSupported("immersive-vr") is false. Flags don't change that, and neither does --force-webxr-runtime=openxr with SteamVR exposed to the Flatpak.
• The binary has no OpenXR loader: no XR_RUNTIME_JSON, xrGetInstanceProcAddr or XR_LOADER_DEBUG strings. The only OpenXR strings are the chrome://flags entries.

Cause (verified against Chromium source, same date). M154 is the first release that compiles OpenXR on Linux:

• device/vr/buildflags/buildflags.gni adds is_linux to enable_openxr.
• Flathub's tarball sets checkout_openxr = true.
• Flathub's GN args don't turn it off.

But content/services/isolated_xr_device/xr_runtime_provider.cc only creates the OpenXR device under ENABLE_OPENXR && IS_WIN. That's true on 154, 155 and main. Nothing on Linux calls the OpenXR code, so the linker drops it. The rest of the Linux port is in two unmerged CLs (bug 506004811):

• 8441736 runs the XR device service in a Linux sandbox that allows SteamVR's sockets, /dev/shm and flock.
• 8132979 wires the provider to OpenXrPlatformHelperLinux. kOpenXR stays off by default, so it needs --enable-features=OpenXR.

8132979 sits on top of 8441736, so fetching refs/changes/79/8132979/<ps> gets both.

The Frame side is ready. ~/.config/openxr/1/active_runtime.json names SteamVR (bin/linuxarm64/vrclient.so, VALVE_runtime_is_steamvr). The Linux backend uses Vulkan (XR_USE_GRAPHICS_API_VULKAN).

Building and installing it

Follow the public repo's README. In short: build/build.sh on an x64 Linux host (no sudo, about 90 GB of disk) produces chromium-xr-arm64.tar.xz (about 145 MB), and frame/install.sh on the Frame unpacks it to ~/chromium-xr, installs the chromium-xr launcher in ~/.local/bin, and adds the Steam library shortcut through the Steam client's DevTools port, the same way as T3 Code (apks.md). Launching the shortcut gives Chromium its own panel, valve.steam.desktopgame.<appid>, like any other app.

First build, 2026-09-25, on a 12-thread, 31 GB x64 Linux box: 9 h 33 min for 94,835 steps, giving Chromium 156.0.8071.0. A rebuild after a one-file change takes under a minute, plus about 4 minutes to repack.

To debug from the Mac, launch it as a panel with DevTools on the Frame (verified 2026-09-27): scripts/panel-on-frame.sh -- '~/.local/bin/chromium-xr' --remote-debugging-port=9223 URL (panels.md). DevTools has no authentication. It listens on loopback, but with the userspace Tailscale from tailscale.md running, loopback ports are reachable from your tailnet. Close the browser when you're done. Chromium runs one browser per profile, so close the Steam-launched one first or the flag is ignored.

The SO_PEERCRED fix. The XR seccomp policy refuses getsockopt. SteamVR's client calls getsockopt(SOL_SOCKET, SO_PEERCRED) inside xrCreateInstance, so the XR process died with a seccomp crash (arm64 syscall 209). The patch allows that one option. It's needed but not enough: the launcher still turns seccomp off (below), so the patch only matters once that's fixed too.

Seccomp is off. The launcher passes --disable-seccomp-filter-sandbox. With the XR seccomp policy on, SteamVR's client reads /proc/self/status through Chrome's file broker and gets the broker's pid. SteamVR then binds the app to the wrong process ("Unable to init path manager: VRInitError_Init_Internal") and xrCreateInstance fails. The broker can't answer /proc/self for another process, so fixing this needs a change in Chromium's broker client or in the CL. The namespace sandbox stays on, but seccomp is off for every process, so use this profile for VR sites rather than everyday browsing.

Upstream (2026-09-27). CL 8441736 (the XR sandbox) has merged into Chromium, still refusing getsockopt; CL 8132979 is still in review. Valve and the CLs' author are working on Steam Frame support (utzcoz/chromium-webxr-linux#5). Both sandbox problems above, with the patch, are reported in utzcoz/chromium-webxr-linux#7.

Verified 2026-09-26 (Frame BUILD_ID 20260922.6101926, SteamVR 2.17.10, this build):

• isSessionSupported('immersive-vr') is true. The WebXR samples page shows "VR support detected".

• requestSession('immersive-vr') succeeds after the prompt. With a WebGL layer, the first XR frame has a viewer pose with 2 views and a 2880 × 1440 framebuffer (1440 × 1440 per eye).

• SteamVR moves the app from VRApplication_OpenXRInstance to VRApplication_OpenXRScene and gives it scene focus. xrEndFrame submits both projection views, and the compositor receives the 2880 × 1440 scene.

• The OpenXR runtime uses Vulkan (XR_KHR_vulkan_enable2). Chromium's own GPU process uses ANGLE on GL, running on zink over Turnip Vulkan (Adreno 750); Chromium's Vulkan backend is off. That doesn't stop the session.

• Unprivileged user namespaces work (unshare -Ur true), so the namespace sandbox runs without the setuid chrome_sandbox.

• With the headset on (same day, seccomp sandbox off): the WebXR samples' Immersive VR Session showed its scene in the headset, and SteamVR loaded the Frame controller bindings for the app. The three.js webxr_vr_video demo, a stereo 360 video, played in 3D after pressing Enter VR.

• Launched from the Steam library, verified remotely 2026-09-27 with nobody wearing the headset (standby workaround in how-the-frame-works.md). The installer's Steam shortcut starts Chromium, and SteamVR takes it as scene app steam.app.<shortcut id>. A minimal WebXR session that clears every frame to red ran at about 75 frames per second, and the stereo headset capture showed both eyes solid red. Steam preloads its overlay (gameoverlayrenderer.so), which crashed Chromium's zygote about 30 s after a Steam launch. The public repo's launcher now removes it from LD_PRELOAD. With the headset outside its playspace, SteamVR shows passthrough wherever the page leaves transparent pixels.

• Frame rate and input, measured 2026-09-27 (standby workaround, red test session): 72 fps with every frame at 13.9–14 ms over 16 s, and SteamVR dropped frames only at startup. The right controller showed up as an oculus-touch tracked-pointer with an xr-standard gamepad and a 25-joint hand, with poses on every frame. A real squeeze reached the page as squeezestart/squeeze. Haptics aren't exposed (no actuators). Details are in the public repo's technical notes.

Not verified yet: trigger, thumbstick and face buttons, the left controller, bare-hand tracking, and third-party VR180 players (DeoVR and DL8 web embeds).