Wayland: Fix window opacity on older systems

On systems lacking the EGL_EXT_present_opaque extension, some
compositors treat any buffer with an alpha channel as per-pixel
transparent.

This commit ignores any EGLConfig with an alpha channel if the extension
is missing and the window is created with GLFW_TRANSPARENT_FRAMEBUFFER
set to false.

This is technically not a breaking change since GLFW_ALPHA_BITS is not
a hard constraint, but it is still going to inconvenience anyone using
the framebuffer alpa channel to store other kinds of data.

Related to #1895

(cherry picked from commit ef6c9d8b4f)

CONTRIBUTORS.md

@@ -180,6 +180,7 @@ video tutorials.
  - pthom
  - Martin Pulec
  - Guillaume Racicot
+ - Christian Rauch
  - Philip Rideout
  - Eddie Ringle
  - Max Risuhin

README.md

@@ -124,6 +124,8 @@ information on what to include when reporting a bug.
 ## Changelog
 
  - [Wayland] Bugfix: Connecting a mouse after `glfwInit` would segfault (#1450)
+ - [Wayland] Disabled alpha channel for opaque windows on systems lacking
+   `EGL_EXT_present_opaque` (#1895)
  - [Linux] Bugfix: Joysticks without buttons were ignored (#2042,#2043)
    [EGL] Added loading of glvnd `libOpenGL.so.0` where available for OpenGL
  - [GLX] Added loading of glvnd `libGLX.so.0` where available

docs/news.dox

@@ -312,6 +312,19 @@ Starting with GLFW 3.3.7, events posted with @ref glfwPostEmptyEvent now use a s
 unnamed pipe instead of sending an X11 client event to the helper window.
 
 
+@subsubsection wayland_alpha_34 Frambuffer may lack alpha channel on older Wayland systems
+
+On Wayland, when creating an EGL context on a machine lacking the new
+`EGL_EXT_present_opaque` extension, the @ref GLFW_ALPHA_BITS window hint will be
+ignored and the framebuffer will have no alpha channel.  This is because some
+Wayland compositors treat any buffer with an alpha channel as per-pixel
+transparent.
+
+If you want a per-pixel transparent window, see the
+[GLFW_TRANSPARENT_FRAMEBUFFER](@ref GLFW_TRANSPARENT_FRAMEBUFFER_hint) window
+hint.
+
+
 @subsection deprecations_33 Deprecations in version 3.3
 
 @subsubsection charmods_callback_33 Character with modifiers callback

src/egl_context.c

@@ -172,6 +172,18 @@ static GLFWbool chooseEGLConfig(const _GLFWctxconfig* ctxconfig,
         u->depthBits = getEGLConfigAttrib(n, EGL_DEPTH_SIZE);
         u->stencilBits = getEGLConfigAttrib(n, EGL_STENCIL_SIZE);
 
+#if defined(_GLFW_WAYLAND)
+        // NOTE: The wl_surface opaque region is no guarantee that its buffer
+        //       is presented as opaque, if it also has an alpha channel
+        // HACK: If EGL_EXT_present_opaque is unavailable, ignore any config
+        //       with an alpha channel to ensure the buffer is opaque
+        if (!_glfw.egl.EXT_present_opaque)
+        {
+            if (!desired->transparent && u->alphaBits > 0)
+                continue;
+        }
+#endif // _GLFW_WAYLAND
+
         u->samples = getEGLConfigAttrib(n, EGL_SAMPLES);
         u->doublebuffer = desired->doublebuffer;