For the last couple of months I've had the issue that the ALT+Tab switcher fails to render with compositing enabled, see [1]. I've finally invested some time to attempt to debug this, but have not come very far.

What I found out:

- When triggering the task-switcher, kwin reports
    QGLXContext: Failed to create dummy context

- Looking at the qt-sources, I found that QT_OPENGL_NO_SANITY_CHECK disables the sanity checking that triggers the dummy context creation.

- So trying with QT_OPENGL_NO_SANITY_CHECK=1 kwin_x11, triggering the task-switcher I get loads of
   QOpenGLShader::compile(Fragment): 0:1(10): error: GLSL 1.50 is not supported. Supported versions are: 1.10, 1.20, 1.30, 1.00 ES, 3.00 ES, 3.10 ES, and 3.20 ES

Somehow it looks like kwin is not using a core GL context with sufficiently high version? My graphics setup is

    Vendor: Intel Open Source Technology Center (0x8086)
    Device: Mesa DRI Intel(R) HD Graphics 530 (Skylake GT2)  (0x191b)
    Version: 17.2.0
    Accelerated: yes
    Video memory: 3072MB
    Unified memory: yes
    Preferred profile: core (0x1)
    Max core profile version: 4.5
    Max compat profile version: 3.0
    Max GLES1 profile version: 1.1
    Max GLES[23] profile version: 3.2


[1] https://smani.fedorapeople.org/taskswitcher.jpg 


Using kwin-5.10.5-1.fc28.x86_64 on Fedora Rawhide