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

CONTRIBUTORS.md

@@ -185,6 +185,7 @@ video tutorials.
  - pthom
  - Martin Pulec
  - Guillaume Racicot
+ - Christian Rauch
  - Philip Rideout
  - Eddie Ringle
  - Max Risuhin

README.md

@@ -315,6 +315,8 @@ information on what to include when reporting a bug.
  - [Wayland] Added support for key names via xkbcommon
  - [Wayland] Added support for file path drop events (#2040)
  - [Wayland] Added support for more human-readable monitor names where available
+ - [Wayland] Disabled alpha channel for opaque windows on systems lacking
+   `EGL_EXT_present_opaque` (#1895)
  - [Wayland] Removed support for `wl_shell` (#1443)
  - [Wayland] Bugfix: The `GLFW_HAND_CURSOR` shape used the wrong image (#1432)
  - [Wayland] Bugfix: `CLOCK_MONOTONIC` was not correctly enabled

docs/news.dox

@@ -126,6 +126,19 @@ To work around this, call any joystick function before waiting for events, for
 example by setting a [joystick callback](@ref joystick_event).
 
 
+@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.
+
+
 @subsubsection standalone_34 Tests and examples are disabled when built as a subproject
 
 GLFW now does not build the tests and examples when it is added as

src/egl_context.c

@@ -173,6 +173,21 @@ static GLFWbool chooseEGLConfig(const _GLFWctxconfig* ctxconfig,
         u->depthBits = getEGLConfigAttrib(n, EGL_DEPTH_SIZE);
         u->stencilBits = getEGLConfigAttrib(n, EGL_STENCIL_SIZE);
 
+#if defined(_GLFW_WAYLAND)
+        if (_glfw.platform.platformID == GLFW_PLATFORM_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;