Created attachment 98089 [details] Demonstration app showing the issue I'm not sure if this is a bug in Mesa or in Qt. But since the behaviour differs depending on whether LIBGL_ALWAYS_SOFTWARE is set or not I think something might be strange in Mesa, which is why I'm filing this here first. I have reduced the problem to a very simple Qt5 application (which I will attach). It has a QGLWidget, which simply calls glClear. When I run the application, a window (with no decorations) seems to appear over part of the screen, at the same size as the widget. It initially shows through to whatever was on the desktop behind it, but the contents are not updated. Additionally, the GL widget does not seem to be filled in: it starts with whatever the background was, and doesn't update. However, if I set LIBGL_ALWAYS_SOFTWARE=1, then it correctly shows as pink (however, the other symptom remains). System information: OS: Xubuntu 14.04 CPU: i5-2430M glxinfo, without environment variables: OpenGL vendor string: Intel Open Source Technology Center OpenGL renderer string: Mesa DRI Intel(R) Sandybridge Mobile OpenGL core profile version string: 3.1 (Core Profile) Mesa 10.1.0 OpenGL core profile shading language version string: 1.40 glxinfo, with LIBGL_ALWAYS_SOFTWARE=1 OpenGL vendor string: VMware, Inc. OpenGL renderer string: Gallium 0.4 on llvmpipe (LLVM 3.4, 256 bits) OpenGL version string: 2.1 Mesa 10.1.0 OpenGL shading language version string: 1.30 I've also tried using Mesa git (151a20d), configured with --with-dri-drivers=i965,swrast --with-gallium-drivers= which showed the same symptoms.
Created attachment 98090 [details] Screenshot when not setting LIBGL_ALWAYS_SOFTWARE Screenshot without setting any environment variables. What looks like a bit of the GIMP and of Bugzilla is actually the application, which has not updated its backbuffer.
Created attachment 98091 [details] Screenshot with LIBGL_ALWAYS_SOFTWARE
I've also discovered that if I disable the compositor in Settings -> Window manager tweaks -> Compositor, then the extra window thing goes away, but I still have the bug that the hardware renderer does not clear the window correctly.
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