Task Switcher Text Rendering Issue on PineTab2 with Wayland

[fromdot]

Context

• I think this problem is related to Task switcher doesn't work on Plasma Mobile #617 (Task switcher black screen issue on PinePhone Pro, resolved with qt6-base-es2)
• Both issues appear to involve Qt6's rendering on Mali GPUs under Wayland, though with different symptoms

Device info

• Device: PineTab2
• Kernel Version : 6.12.3-danctnix1-1-pinetab2
• UI: Plasma Desktop (KDE Plasma 6.2.4)
• Qt version: 6.8.1
• Session: Wayland
• GPU: Mali-G52 with Panfrost driver

Describe the bug
In Wayland session, the Task Switcher displays text as white boxes. Each character in window titles is rendered as a separate white box with size proportional to the character's dimensions. The issue occurs across all Task Switcher layouts (sidebar, compact, thumbnail_grid, etc.).

The issue only affects Task Switcher text; other UI text renders normally. The problem does not occur in X11 sessions.

Attempted Solutions

1. Environment variables (none worked):

   • QT_WAYLAND_FORCE_DUMB_RENDERING=1
   • QT_QUICK_BACKEND=software
   • LIBGL_ALWAYS_SOFTWARE=1
   • Several Qt font rendering related settings (QT_FONT_RENDER_RULE, QT_XFT_ANTIALIAS, etc.)

2. KWin configuration:

   • Modified compositor backend to XRender
   • Disabled GLCore
   • Tried different Task Switcher layouts (sidebar, compact, thumbnail_grid, etc.)

Solution/Workaround
Adding renderType: Text.NativeRendering to the Kirigami.Heading component in /usr/share/kwin/tabbox/sidebar/contents/ui/main.qml resolves the issue:

Kirigami.Heading {
    Layout.fillWidth: true
    height: undefined
    level: 4
    text: model.caption
    elide: Text.ElideRight
    wrapMode: Text.WrapAtWordBoundaryOrAnywhere
    maximumLineCount: 2
    lineHeight: 0.95
    textFormat: Text.PlainText
    renderType: Text.NativeRendering  // Added this line
}

Screenshots
Before:

After:

Logfiles and additional information

journalctl -b | grep -i kwin_wayland

Key OpenGL errors observed:

kwin_scene_opengl: 0x3: GL_INVALID_OPERATION in glDrawBuffers(unsupported buffer GL_BACK_LEFT)
kwin_scene_opengl: 0x3: GL_INVALID_OPERATION in glTexSubImage2D(invalid texture level 0)
kwin_scene_opengl: 0x3: GL_INVALID_OPERATION in glDrawElements
kwin_scene_opengl: 0x3: GL_INVALID_OPERATION in glVertexAttribPointer(no array object bound)

GL_INVALID_VALUE in glTexImage2D(internalFormat=GL_ALPHA)
GL_INVALID_ENUM in glEnable(GL_POINT_SPRITE)

Full kwin_wayland log:
kwin_wayland_log.txt

Note
This issue report was prepared with the assistance of Claude AI, which helped analyze the logs and suggested various debugging approaches. The final solution (adding renderType: Text.NativeRendering) was found through collaborative debugging.

Before submitting a bug report, please be sure that you have done the following:

• I have checked for updates (pacman -Syu) and installed the updates but the problem persisted.
• I am not using Megi's multiboot image, which is unsupported by the developer.