Fix visual selection with EGL on xcb

Using a 24-bit visual for 8888 configs is wrong since we loose the
alpha channel. This breaks translucent windows and, among others,
leads to not showing drag pixmaps (that typically have transparent
areas) properly.

Change-Id: I516c84327680b76996b622831e431c29d840471e
Task-number: QTBUG-35126
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

1 files changed, 14 insertions, 25 deletions

diff --git a/src/platformsupport/eglconvenience/qxlibeglintegration.cpp b/src/platformsupport/eglconvenience/qxlibeglintegration.cpp
index 80453816fc..9c2d50823a 100644
--- a/src/platformsupport/eglconvenience/qxlibeglintegration.cpp
+++ b/src/platformsupport/eglconvenience/qxlibeglintegration.cpp
@@ -84,32 +84,22 @@ VisualID QXlibEglIntegration::getCompatibleVisualId(Display *display, EGLDisplay
             int visualRedSize = qPopulationCount(chosenVisualInfo->red_mask);
             int visualGreenSize = qPopulationCount(chosenVisualInfo->green_mask);
             int visualBlueSize = qPopulationCount(chosenVisualInfo->blue_mask);
-            int visualAlphaSize = -1; // Need XRender to tell us the alpha channel size
-
-            bool visualMatchesConfig = false;
-            if ( visualRedSize == configRedSize &&
-                 visualGreenSize == configGreenSize &&
-                 visualBlueSize == configBlueSize )
-            {
-                // We need XRender to check the alpha channel size of the visual. If we don't have
-                // the alpha size, we don't check it against the EGL config's alpha size.
-                if (visualAlphaSize >= 0)
-                    visualMatchesConfig = visualAlphaSize == configAlphaSize;
-                else
-                    visualMatchesConfig = true;
-            }
+            int visualAlphaSize = chosenVisualInfo->depth == 32 ? 8 : 0;
+
+            const bool visualMatchesConfig = visualRedSize == configRedSize
+                && visualGreenSize == configGreenSize
+                && visualBlueSize == configBlueSize
+                && visualAlphaSize == configAlphaSize;
 
+            // In some cases EGL tends to suggest a 24-bit visual for 8888
+            // configs. In such a case we have to fall back to XGetVisualInfo.
             if (!visualMatchesConfig) {
-                if (visualAlphaSize >= 0) {
-                    qWarning("Warning: EGL suggested using X Visual ID %d (ARGB%d%d%d%d) for EGL config %d (ARGB%d%d%d%d), but this is incompatable",
-                             (int)visualId, visualAlphaSize, visualRedSize, visualGreenSize, visualBlueSize,
-                             configId, configAlphaSize, configRedSize, configGreenSize, configBlueSize);
-                } else {
-                    qWarning("Warning: EGL suggested using X Visual ID %d (RGB%d%d%d) for EGL config %d (RGB%d%d%d), but this is incompatable",
-                             (int)visualId, visualRedSize, visualGreenSize, visualBlueSize,
-                             configId, configRedSize, configGreenSize, configBlueSize);
-                }
                 visualId = 0;
+#ifdef QT_DEBUG_X11_VISUAL_SELECTION
+                qWarning("Warning: EGL suggested using X Visual ID %d (%d %d %d depth %d) for EGL config %d (%d %d %d %d), but this is incompatible",
+                         (int)visualId, visualRedSize, visualGreenSize, visualBlueSize, chosenVisualInfo->depth,
+                         configId, configRedSize, configGreenSize, configBlueSize, configAlphaSize);
+#endif
             }
         } else {
             qWarning("Warning: EGL suggested using X Visual ID %d for EGL config %d, but that isn't a valid ID",
@@ -133,8 +123,7 @@ VisualID QXlibEglIntegration::getCompatibleVisualId(Display *display, EGLDisplay
         return visualId;
     }
 
-    // Finally, try to
-    // use XGetVisualInfo and only use the bit depths to match on:
+    // Finally, try to use XGetVisualInfo and only use the bit depths to match on:
     if (!visualId) {
         XVisualInfo visualInfoTemplate;
         memset(&visualInfoTemplate, 0, sizeof(XVisualInfo));