Full translucent background support in xcb and xlib backend.

Make sure to pick an alpha visual also for non-GL surface types, and to
ask for alpha in the window format if the WA_TranslucentBackground
attribute is set.

Reviewed-by: Janusz Lewandowski
(cherry picked from commit 6241e39cff9311c943430ff2f31236b13618f2ac)

2 files changed, 15 insertions, 5 deletions

diff --git a/src/opengl/qgl_qpa.cpp b/src/opengl/qgl_qpa.cpp
index 994344c6eb..9da650f98d 100644
--- a/src/opengl/qgl_qpa.cpp
+++ b/src/opengl/qgl_qpa.cpp
@@ -150,6 +150,8 @@ bool QGLContext::chooseContext(const QGLContext* shareContext)
             if (shareContext) {
                 winFormat.setSharedContext(shareContext->d_func()->platformContext);
             }
+            if (widget->testAttribute(Qt::WA_TranslucentBackground))
+                winFormat.setAlpha(true);
             winFormat.setWindowApi(QPlatformWindowFormat::OpenGL);
             winFormat.setWindowSurface(false);
             widget->setPlatformWindowFormat(winFormat);
diff --git a/src/opengl/qwindowsurface_gl.cpp b/src/opengl/qwindowsurface_gl.cpp
index 49b3dc2dae..ff5744678d 100644
--- a/src/opengl/qwindowsurface_gl.cpp
+++ b/src/opengl/qwindowsurface_gl.cpp
@@ -542,19 +542,27 @@ void QGLWindowSurface::beginPaint(const QRegion &)
     d_ptr->did_paint = true;
     updateGeometry();
 
-    if (!context())
-        return;
-
     int clearFlags = 0;
 
-    if (context()->d_func()->workaround_needsFullClearOnEveryFrame)
+    QGLContext *ctx = reinterpret_cast<QGLContext *>(window()->d_func()->extraData()->glContext);
+
+    if (!ctx)
+        return;
+
+    if (ctx->d_func()->workaround_needsFullClearOnEveryFrame)
         clearFlags = GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT;
-    else if (context()->format().alpha())
+    else if (ctx->format().alpha())
         clearFlags = GL_COLOR_BUFFER_BIT;
 
     if (clearFlags) {
+        if (d_ptr->fbo)
+            d_ptr->fbo->bind();
+
         glClearColor(0.0, 0.0, 0.0, 0.0);
         glClear(clearFlags);
+
+        if (d_ptr->fbo)
+            d_ptr->fbo->release();
     }
 }