KWin refuses to start with compositor enabled?!

[René J.V. Bertin]

Hi,

I updated my home-built KWin 5.13.3, first to 5.13.3, then to 5.15.5 in hopes of experiencing less compositing glitches (that required me to restart KWin several times a day, ultimately forcing my back to xfwm4).

The jury is still out if there are less frequent glitches (and if those that remain aren't more severe), but now it requires me to activate compositing by hand after each restart, regardless of the corresponding startup setting. Kwin 4 worked fine without compositing, even supported translucency during window moves (and compositor support in apps shouldn't depend on the window manager, right?!) but Kwin 5 can apparently no longer render window shadows without the compositor.And that's a deal breaker.

Below is the output I'm getting after a restart through krunner. The OpenGL output appears when I hit Shift-Alt-F12 to activate compositing:

org.kde.kwindowsystem: Loaded plugin "/opt/local/share/qt5/plugins/kf5/org.kde.kwindowsystem.platforms/KF5WindowSystemX11Plugin.so" for platform "xcb"
kwin_core: XCB error: 147 (BadOutput), sequence: 549, resource id: 2713715, major code: 140 (RANDR), minor code: 9 (GetOutputInfo)
kwin_core: XCB error: 147 (BadOutput), sequence: 550, resource id: 172, major code: 140 (RANDR), minor code: 9 (GetOutputInfo)
qt.qpa.xcb: QXcbConnection: XCB error: 3 (BadWindow), sequence: 3728, resource id: 146800811, major code: 3 (GetWindowAttributes), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 3729, resource id: 146800811, major code: 14 (GetGeometry), minor code: 0
OpenGL vendor string:                   Intel Open Source Technology Center
OpenGL renderer string:                 Mesa DRI Intel(R) HD Graphics 400 (Braswell) 
OpenGL version string:                  4.5 (Core Profile) Mesa 18.3.3
OpenGL shading language version string: 4.50
Driver:                                 Intel
GPU class:                              Unknown
OpenGL version:                         4.5
GLSL version:                           4.50
Mesa version:                           18.3.3
X server version:                       1.18.3
Linux kernel version:                   4.14.23
Requires strict binding:                yes
GLSL shaders:                           yes
Texture NPOT support:                   yes
Virtual Machine:                        no
libkwinglutils: Skipping self test as it is reported to return false positive results on Mesa drivers
kwin_core: XCB error: 10 (BadAccess), sequence: 3803, resource id: 246, major code: 142 (Composite), minor code: 2 (RedirectSubwindows)
qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 3806, resource id: 0, major code: 14 (GetGeometry), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 3816, resource id: 0, major code: 14 (GetGeometry), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 3826, resource id: 0, major code: 14 (GetGeometry), minor code: 0
qt.qpa.xcb: QXcbConnection: XCB error: 9 (BadDrawable), sequence: 3836, resource id: 0, major code: 14 (GetGeometry), minor code: 0