Phase 0: browser X11-overlay inventory + mpv reference cell + tooling installs

Source-level verdict: no browser in the matrix has a code path
to hand NV12 to the X server for plane scanout. Chromium ozone-x11
wires StubOverlayManager (ozone_platform_x11.cc:262); Brave 147 +
chromium-fourier 149 inherit unchanged. Firefox WindowSurfaceX11
is RGB-only. The campaign's load-bearing hypothesis is structurally
weakened — what the X11 cells will measure for browsers is
KWin's per-frame RGB-recomposite cost, not the original
"force-GL-composite of NV12" framing. mpv --vo=xv becomes the
matrix's only direct test of the operator-supplied mechanism.

Matrix updated: 12 cells -> 16 cells (+8 mpv VO sub-points).
revert.log entries 1-5 capture all package + per-user state
mutations from this turn (measurement tools, openbox, XFCE +
xfwm4-no-comp pre-seed, firefox + AUR xtrace, XFCE rotation
+ touchscreen mapping); single SSH-driveable revert chain
returns ohm to its pre-campaign 1169-package state.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

phase0_evidence/chromium_ozone_x11_2026-05-03/ui/ozone/common/stub_overlay_manager.cc

@@ -0,0 +1,24 @@
+// Copyright 2015 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "ui/ozone/common/stub_overlay_manager.h"
+
+#include <memory>
+
+#include "ui/ozone/public/overlay_candidates_ozone.h"
+
+namespace ui {
+
+StubOverlayManager::StubOverlayManager() {
+}
+
+StubOverlayManager::~StubOverlayManager() {
+}
+
+std::unique_ptr<OverlayCandidatesOzone>
+StubOverlayManager::CreateOverlayCandidates(gfx::AcceleratedWidget w) {
+  return nullptr;
+}
+
+}  // namespace ui

phase0_evidence/chromium_ozone_x11_2026-05-03/ui/ozone/common/stub_overlay_manager.h

@@ -0,0 +1,30 @@
+// Copyright 2015 The Chromium Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef UI_OZONE_COMMON_STUB_OVERLAY_MANAGER_H_
+#define UI_OZONE_COMMON_STUB_OVERLAY_MANAGER_H_
+
+#include <memory>
+
+#include "ui/ozone/public/overlay_manager_ozone.h"
+
+namespace ui {
+
+class StubOverlayManager : public OverlayManagerOzone {
+ public:
+  StubOverlayManager();
+
+  StubOverlayManager(const StubOverlayManager&) = delete;
+  StubOverlayManager& operator=(const StubOverlayManager&) = delete;
+
+  ~StubOverlayManager() override;
+
+  // OverlayManagerOzone:
+  std::unique_ptr<OverlayCandidatesOzone> CreateOverlayCandidates(
+      gfx::AcceleratedWidget w) override;
+};
+
+}  // namespace ui
+
+#endif  // UI_OZONE_COMMON_STUB_OVERLAY_MANAGER_H_