Switch the headful window manager from mutter to openbox

[chruffins]

Summary

Swaps the headful X11 window manager from mutter to openbox.

Mutter composites every frame Chromium presents. On non-GPU browsers, that compositing runs on the CPU and adds significant overhead under rendering load. Openbox is a stacking, non-compositing WM that provides what the headful image needs (decorations, focus, stacking, popup titlebars) without compositing.

Benchmark

Measured on a private build of this change, not re-run on this repo's tree. 8 vCPU / 8 GiB / 2 GiB /dev/shm / 1920×1080, --disable-gpu --use-angle=swiftshader. Medians of 3 trials per arm (1 for idle), order-balanced, same image and flags in both arms. Per-process CPU from /proc/<pid>/stat tick deltas at 1 Hz. All figures are percent of one CPU.

Workload	Metric	mutter	openbox
Busy	display stack total	95.4%	1.9%
Busy	window manager	87.9%	0.00%
Busy	chromium total	141.6%	145.3%
Busy	screenshot latency p50 / p95	217 / 276 ms	165 / 212 ms
Idle	display stack	10.0%	0.1%
Live view	display stack	70.9%	3.7%

Chromium's CPU and frame output are unchanged across arms, so this removes overhead without skipping work.

Changes

• Install openbox instead of mutter. Add x11-utils for xprop.
• Ship a kernel-light openbox theme and rc.xml at /etc/xdg/openbox/. Titlebars are close-only. Minimize would unmap a popup with no taskbar to restore it.
• Paint the root window with the previous gradient (shared/set-desktop-background.sh) before the WM starts. Every stage is bounded by timeout, so a wedged X call can't stall startup.
• x11.WaitForWindowManager reads _NET_SUPPORTING_WM_CHECK (EWMH) instead of searching for a process name, so it works for any conforming WM.
• Update mutter references in comments, the e2e test, and the crashed-service example in the API spec (openapi.yaml, plus the matching description in oapi.go).

Testing

• go vet ./... and go test -race across the server module, e2e excluded.
• Headful image builds from this branch. Smoke run of the built image: openbox and chromium are RUNNING under supervisord, _NET_SUPPORTING_WM_CHECK is set on the root window, the background gradient samples at the expected corner colors (#123250, #1a4a79), and CDP /json/version responds.
• Not run yet: the headful e2e suite. It needs to pass before merge.

Known behavior differences

• Fullscreen: Chromium reports window bounds of 2048×1536 under openbox versus 3840×2160 under mutter. The X root size is correct in both. openbox doesn't transiently expand the root during fullscreen. The kiosk e2e case covers fullscreen tracking.
• Some window.open sequences can open popups stacked under the opener. This is identical under mutter and predates this change.
• The background is painted at boot only. A later display resize may leave the root unpainted until the next boot. Not verified.

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Note

Medium Risk
Changes the headful display boot sequence and window-manager behavior (CSD, resize reflow, live view CPU), though readiness is now WM-agnostic and startup paths are timeout-bounded.

Overview
Replaces mutter with openbox in the headful Chromium image to avoid CPU compositing overhead while keeping decorations, focus, and popup titlebars.

The image now installs openbox (plus x11-utils for xprop), ships kernel-light theme and /etc/xdg/openbox/rc.xml, and supervisord runs openbox instead of mutter. Before the WM starts, the wrapper runs set-desktop-background.sh to paint the blue gradient on the X root; each stage is timeout-bounded so wedged X probes cannot block boot.

x11.WaitForWindowManager replaces mutter-specific polling with an EWMH _NET_SUPPORTING_WM_CHECK probe so chromium-launcher and the wrapper work with any conforming WM. Comments, e2e resize tests, and telemetry/OpenAPI examples are updated from mutter to openbox/generic “window manager”; unit tests cover supportingWindowID parsing and stalled background-script probes.

^{Reviewed by Cursor Bugbot for commit 3dc8a83. Configure here.}