Disable surfaceless QOffscreenSurface with Mesa

With Intel at least Mesa is unable to handle surfaceless contexts in
glReadPixels(). This cripples QOpenGLFramebufferObject::toImage() and
potentially others too.

Task-number: QTBUG-46605
Change-Id: I07c1015eca67b8add14496ec0df0e0c17ac3d896
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>

1 files changed, 12 insertions, 1 deletions

diff --git a/src/platformsupport/eglconvenience/qeglpbuffer.cpp b/src/platformsupport/eglconvenience/qeglpbuffer.cpp
index 9cdf5a0931..756609a641 100644
--- a/src/platformsupport/eglconvenience/qeglpbuffer.cpp
+++ b/src/platformsupport/eglconvenience/qeglpbuffer.cpp
@@ -55,7 +55,18 @@ QEGLPbuffer::QEGLPbuffer(EGLDisplay display, const QSurfaceFormat &format, QOffs
     , m_display(display)
     , m_pbuffer(EGL_NO_SURFACE)
 {
-    if (q_hasEglExtension(display, "EGL_KHR_surfaceless_context"))
+    bool hasSurfaceless = q_hasEglExtension(display, "EGL_KHR_surfaceless_context");
+
+    // Disable surfaceless contexts on Mesa for now. As of 10.6.0 and Intel at least, some
+    // operations (glReadPixels) are unable to work without a surface since they at some
+    // point temporarily unbind the current FBO and then later blow up in some seemingly
+    // safe operations, like setting the viewport, that apparently need access to the
+    // read/draw surface in the Intel backend.
+    const char *vendor = eglQueryString(display, EGL_VENDOR); // hard to check for GL_ strings here, so blacklist all Mesa
+    if (vendor && strstr(vendor, "Mesa"))
+        hasSurfaceless = false;
+
+    if (hasSurfaceless)
         return;
 
     EGLConfig config = q_configFromGLFormat(m_display, m_format, false, EGL_PBUFFER_BIT);