KWIN_PIVOT: Phase-2 findings — bug is in Qt 6, not KWin

Source-grep collapsed Phase 1+2 onto a single pass. KWin's own GL paths
use GL_R8 correctly (gltexture.cpp:61, shadowitem.cpp:494). The
glTexImage2D(GL_ALPHA) calls observed in the journal originate from
Qt 6:

- qtbase/src/opengl/qopengltextureglyphcache.cpp:111-117 — text glyph
  cache upload path. The #else branch (active when qtbase is built
  with QT_CONFIG(opengles2)) unconditionally uses GL_ALPHA, with no
  runtime check for ES context major version. Correct on ES 2.x;
  broken on ES 3.x where GL_ALPHA is no longer a valid glTexImage2D
  internalFormat.
- qtbase/src/gui/rhi/qrhigles2.cpp:1373-1378 — Qt-Quick-RHI sibling.
  Same logic, gated only on caps.coreProfile, missing the ES≥3 case.
- qtbase/src/opengl/qopengltextureuploader.cpp:253-257 — QImage→GL
  upload path; same shape.

KWin runs an ES 3.2 context on Mali-G52 panfrost (RK3566), Qt picks
GL_ALPHA, mesa returns GL_INVALID_VALUE, every dependent draw errors
at level 0, the compositor's frame-callback path stalls. KWin is the
visible victim because it's the compositor, but the bug is in Qt.

KWIN_PIVOT.md rewritten: the patch series and packaging now target
qt6-base-fourier instead of kwin-fourier. Three small hunks (~3 lines
each), runtime-safe via existing caps.gles + caps.ctxMajor / surface
format majorVersion checks. Upstream landing path: bugreports.qt.io
+ Gerrit change against qtbase dev branch.

arch/chromium-fourier/KWIN_PIVOT.md

@@ -1,5 +1,30 @@
 # KWin pivot — fix the `glTexImage2D(GL_ALPHA)` stall
 
+> **2026-04-28 update — Phase 2 collapsed onto Phase 1: it's not KWin.**
+> Source-grep nailed the offender on the first pass. Real culprit:
+> Qt 6's `QOpenGLTextureGlyphCache` (`src/opengl/qopengltextureglyphcache.cpp:111-117`)
+> and `QRhiGles2::toGlTextureFormat` (`src/gui/rhi/qrhigles2.cpp:1373-1378`).
+> KWin's own GL paths use `GL_R8` correctly (`src/opengl/gltexture.cpp:61`,
+> `src/scene/shadowitem.cpp:494`). The pivot becomes a **Qt-fourier**
+> patch, not a kwin-fourier one. Plan rewritten below; the pre-rewrite
+> reproduction/triangulation phases are kept verbatim because they
+> still apply to whatever lives downstream of the Qt fix.
+>
+> Qt's broken logic, in plain English: *"If qtbase was built with
+> opengles2, just always use `GL_ALPHA`."* That's correct for an
+> OpenGL ES 2.x context. It's wrong for OpenGL ES 3.x, where
+> `GL_ALPHA` is no longer a valid `glTexImage2D` internalFormat
+> (only sized formats — `GL_R8`, etc.). Mali / panfrost on RK3566
+> exposes ES 3.2; KWin requests an ES 3.2 context; Qt picks
+> `GL_ALPHA`; mesa returns `GL_INVALID_VALUE`; the texture is
+> permanently broken; every dependent draw errors at level 0; the
+> compositor's frame-callback path stalls. Affects every Qt 6
+> application on Mali-class hardware that ends up rendering text
+> through `QOpenGLTextureGlyphCache` (KDE's window decorations,
+> Plasma overlays, Qt Quick scenegraph via RHI, ad nauseam) — KWin
+> just happens to be the most visible victim because it's the
+> compositor and its stall takes everyone else down with it.
+
 ## What we know
 
 KWin 6.6.4-1 on Arch Linux ARM (Plasma 6.6.4-1, mesa 26.0.5-1, libdrm
@@ -118,50 +143,124 @@ Tooling to identify *which*:
 
 ## Phase 3 — Write the patch (½ evening once Phase 2 is done)
 
-If the offender is a `GL_ALPHA` allocation in a GLES3 context, the
-fix is mechanical:
+The Qt 6 fix is two ~3-line changes, runtime-safe, no new dependency.
+
+**Fix #1 — `src/opengl/qopengltextureglyphcache.cpp` lines 111-117:**
 
 ```diff
-- glTexImage2D(GL_TEXTURE_2D, 0, GL_ALPHA, width, height, 0,
--              GL_ALPHA, GL_UNSIGNED_BYTE, data);
-+ glTexImage2D(GL_TEXTURE_2D, 0, GL_R8, width, height, 0,
-+              GL_RED, GL_UNSIGNED_BYTE, data);
+ #if !QT_CONFIG(opengles2)
+     const GLint internalFormat = isCoreProfile() ? GL_R8 : GL_ALPHA;
+     const GLenum format = isCoreProfile() ? GL_RED : GL_ALPHA;
+ #else
+-    const GLint internalFormat = GL_ALPHA;
+-    const GLenum format = GL_ALPHA;
++    // OpenGL ES 3.x deprecated GL_ALPHA as a glTexImage2D
++    // internalFormat; only true ES 2 contexts retain it. Use GL_R8
++    // + the matching swizzle (handled in the fragment shader's .r
++    // sample below) on ES 3+ hardware so Mali / panfrost / panthor
++    // GLES3 contexts stop emitting GL_INVALID_VALUE every frame.
++    const bool useR8 = ctx->format().majorVersion() >= 3;
++    const GLint internalFormat = useR8 ? GL_R8 : GL_ALPHA;
++    const GLenum format = useR8 ? GL_RED : GL_ALPHA;
+ #endif
 ```
 
-…and adjust the consuming shader's swizzle:
+The downstream fragment shader path that samples this texture must
+read `.r` instead of `.a` when `GL_R8` is used. Qt's text-rendering
+fragment program already has both code paths conditioned on context
+core-profile; the ES 3+ branch needs the same treatment. Lines
+214-216 of the same file (the resize / re-upload path) need the
+identical change.
+
+**Fix #2 — `src/gui/rhi/qrhigles2.cpp` lines 1373-1378:**
 
 ```diff
-- gl_FragColor = vec4(texture2D(s, uv).a, …);
-+ gl_FragColor = vec4(texture2D(s, uv).r, …);
+     case QRhiTexture::RED_OR_ALPHA8:
+-        *glintformat = caps.coreProfile ? GL_R8 : GL_ALPHA;
++        *glintformat = (caps.coreProfile || (caps.gles && caps.ctxMajor >= 3))
++                       ? GL_R8 : GL_ALPHA;
+         *glsizedintformat = *glintformat;
+-        *glformat = caps.coreProfile ? GL_RED : GL_ALPHA;
++        *glformat = (caps.coreProfile || (caps.gles && caps.ctxMajor >= 3))
++                    ? GL_RED : GL_ALPHA;
+         *gltype = GL_UNSIGNED_BYTE;
+         break;
 ```
 
-If the offender is a per-plane fallback in the dmabuf import path
-(suspect #1 above), the patch is larger because the right fix is to
-*not fall through to the broken path* — handle the `external_only`
-case by binding `GL_TEXTURE_EXTERNAL_OES` instead. That mirrors the
-chromium-fourier patch 3/3 done at the chromium layer; symmetry says
-KWin should do the same in its `glTexImage` consumer.
+`caps.gles` and `caps.ctxMajor` are populated at context creation
+(qrhigles2.cpp:804 + :855); the disjunct is free.
+
+**Fix #3 — `src/opengl/qopengltextureuploader.cpp` lines 253-257:**
+
+This is the QImage→GL upload path (used by `QOpenGLPaintEngineEx`
+and its descendants). Same pattern, same fix shape: extend the
+"core profile or GLES2 fallback" branching to also consider GLES3+
+as needing `GL_R8`.
+
+If we want to be aggressive, we can collapse all three sites onto a
+single `qt_gl_use_r8_for_alpha8(ctx)` helper in `qopenglhelper_p.h`
+so future Qt versions don't drift apart again — but a minimal patch
+should keep the three sites independent so each is reviewable in
+isolation by the relevant Qt module owner.
 
 ## Phase 4 — Ship and upstream (1 evening)
 
-1. **Local Arch package** as `kwin-fourier` under
-   `marfrit-packages/arch/kwin-fourier/`, sibling to chromium-fourier
-   and firefox-fourier. PKGBUILD inherits from `extra/kwin`, drops
-   in our patch, bumps `pkgrel`. Same `provides=kwin conflicts=kwin`
-   pattern.
-2. **Validate on ohm** by running the chromium-fourier 149-r2 build +
-   the bbb sample for a minute uninterrupted. Success = no GL_ALPHA
-   in the journal, no stall, smooth playback at the 34.7 % CPU
-   number from the chromium validation.
+1. **Local Arch package** as `qt6-base-fourier` under
+   `marfrit-packages/arch/qt6-base-fourier/`, sibling to chromium-fourier
+   and firefox-fourier. PKGBUILD inherits from `extra/qt6-base`, drops
+   in the three patches above, bumps `pkgrel`. Same
+   `provides=qt6-base conflicts=qt6-base` pattern. Rebuild is heavy
+   (qtbase compile is ~30 minutes on boltzmann; ohm rebuild is
+   sustained-fan-territory and probably better avoided — boltzmann
+   builds the aarch64 .pkg.tar.zst, then we rsync it to ohm and
+   `pacman -U` there).
+2. **Validate on ohm** by:
+   - `pacman -U` the patched qt6-base.
+   - Restart Plasma session (logout / login) so the new qt6-base.so
+     is mapped into the fresh kwin_wayland.
+   - Re-run `journalctl -u plasma-kwin_wayland.service -f` while
+     opening any Qt 6 application that triggers text caching (a
+     terminal, kate, the system tray) — the GL_INVALID_VALUE spam
+     should be **gone**.
+   - Then run chromium-fourier 149-r2 + the bbb sample for a full
+     minute uninterrupted. Success = smooth playback through to EOF
+     at the 34.7 % CPU number, no stall, no audio static, no
+     KWin-side errors in the journal.
 3. **Upstream** via:
-   - File a `kwin` bug on bugs.kde.org with: apitrace fragment, our
-     hardware (Mali-G52 panfrost on RK3566 mainline), exact mesa
-     version, repro steps via `weston-simple-dmabuf-egl` if Phase 1
-     produced one.
-   - Push an MR to invent.kde.org/plasma/kwin against `master`.
-4. **Document** the fix in `chromium-fourier/docs/dmabuf-zero-copy.md`
-   so the next person who lands on the same wall finds the breadcrumb
-   trail.
+   - File on `bugreports.qt.io` against `QtBase: OpenGL`, with: the
+     three diff hunks above, the exact behavior on Mali-G52 panfrost
+     RK3566 mainline 6.19, an excerpt of the journal noise, and
+     mesa 26.0.5 / qt 6.11.0 / kwin 6.6.4 versions.
+   - Push a Gerrit change against `qtbase` `dev` branch
+     (`codereview.qt-project.org`). Qt won't accept a GitHub MR —
+     they live on Gerrit. Create a Qt account, configure
+     `git-review`, push.
+   - Reference the chromium-fourier project as the discovery site
+     so the next Mali-on-Linux Qt 6 user finds the breadcrumb.
+4. **Document** the fix in
+   `chromium-fourier/docs/dmabuf-zero-copy.md` "Caveat — KWin 6.6.4
+   GLES backend on this hardware" subsection: replace the "to be
+   investigated" wording with "fixed by qt6-base-fourier; see
+   `marfrit-packages/arch/qt6-base-fourier/`. Upstream Qt change
+   pending review at `<gerrit-link>`."
+
+## Reflection — corporate IT spec leakage, as predicted
+
+The user's Phase-1 hypothesis was that this was the result of code
+written by people who never read the spec they were claiming to
+implement. They were correct, with one nuance: the Qt code did read
+the spec — *the OpenGL ES 2.x spec*, where `GL_ALPHA` is genuinely
+the canonical single-channel format for `glTexImage2D`. What it
+never went back and re-read is the OpenGL ES 3.0 spec
+(section 3.8.3, "Texture Image Specification"), where `GL_ALPHA`
+is moved to the deprecated list and only sized formats are
+retained. The bug is: *Qt 6 was written assuming "OpenGL ES" is
+one thing, and never updated the assumption when ES 3 dropped the
+unsized formats.* That's a corporate-IT-style architectural
+shortcut: codify the world in two boxes (desktop vs ES), call it
+done, ship. The fact that a category had a sub-category which moved
+in 2012 is not the framework's job to track. Until the bug report
+arrives and someone has to extend the boolean to a triple.
 
 ## What success looks like