Handle when XVisuals lose the alpha channel of the FBConfig

Sometimes the XVisual for an FBConfig have no alpha data, and thus
won't work when an alpha channel is required.

Fixes: QTBUG-74578
Change-Id: Idf05cbfcaea5edf667035939e9bc5d5df2172eec
Reviewed-by: Laszlo Agocs <[email protected]>

1 files changed, 3 insertions, 1 deletions

diff --git a/src/platformsupport/glxconvenience/qglxconvenience.cpp b/src/platformsupport/glxconvenience/qglxconvenience.cpp
index 40521ef6da3..6458454336e 100644
--- a/src/platformsupport/glxconvenience/qglxconvenience.cpp
+++ b/src/platformsupport/glxconvenience/qglxconvenience.cpp
@@ -223,6 +223,7 @@ GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format
                     continue;
             }
 
+            QXlibPointer<XVisualInfo> visual(glXGetVisualFromFBConfig(display, candidate));
             int actualRed;
             int actualGreen;
             int actualBlue;
@@ -231,7 +232,8 @@ GLXFBConfig qglx_findConfig(Display *display, int screen , QSurfaceFormat format
             glXGetFBConfigAttrib(display, candidate, GLX_GREEN_SIZE, &actualGreen);
             glXGetFBConfigAttrib(display, candidate, GLX_BLUE_SIZE, &actualBlue);
             glXGetFBConfigAttrib(display, candidate, GLX_ALPHA_SIZE, &actualAlpha);
-
+            // Sometimes the visuals don't have a depth that includes the alpha channel.
+            actualAlpha = qMin(actualAlpha, visual->depth - actualRed - actualGreen - actualBlue);
 
             if (requestedRed && actualRed < requestedRed)
                 continue;