Fix up QOpenGLWidget transparency support

The glColorMask call was troublesome. In addition, the Qt::WA_TranslucentBackground
was misinterpreted and recommended misleadingly in the documentation. The hellogl2
example's --transparent argument was disfunctional in practice.

Replace glColorMask with glBlendFuncSeparate. The hellogl2 example and the docs are
now corrected wrt enabling semi-transparency in a QOpenGLWidget that is not a top-level
(which is the most common case).

Task-number: QTBUG-47276
Change-Id: I6f40e732d455f5efcf158649ac9a52ff9f240e85
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@theqtcompany.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>

1 files changed, 1 insertions, 6 deletions

diff --git a/src/gui/painting/qplatformbackingstore.cpp b/src/gui/painting/qplatformbackingstore.cpp
index e765a9e402..c86fdebea5 100644
--- a/src/gui/painting/qplatformbackingstore.cpp
+++ b/src/gui/painting/qplatformbackingstore.cpp
@@ -300,11 +300,7 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
     }
 
     funcs->glEnable(GL_BLEND);
-    funcs->glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
-    // Do not write out alpha. We need blending, but only for RGB. The toplevel may have
-    // alpha enabled in which case blending (writing out < 1.0 alpha values) would lead to
-    // semi-transparency even when it is not wanted.
-    funcs->glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_FALSE);
+    funcs->glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE);
 
     // Backingstore texture with the normal widgets.
     GLuint textureId = 0;
@@ -364,7 +360,6 @@ void QPlatformBackingStore::composeAndFlush(QWindow *window, const QRegion &regi
             blit(textures, i, window, deviceWindowRect, d_ptr->blitter);
     }
 
-    funcs->glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
     funcs->glDisable(GL_BLEND);
     d_ptr->blitter->release();