GtkGLArea and libmpv need compatible rendering paths. Selecting GLES or
Vulkan can break composition on proprietary NVIDIA Wayland drivers.
Default GDK and GSK to desktop OpenGL before GTK starts

diff --git a/src/main.rs b/src/main.rs
--- a/src/main.rs
+++ b/src/main.rs
@@ -33,6 +33,18 @@
 }
 
 fn main() -> ExitCode {
+    // GtkGLArea and libmpv must use desktop OpenGL.  In particular, allowing
+    // GDK to choose GLES or GSK to choose Vulkan breaks composition on the
+    // proprietary NVIDIA Wayland driver.  Keep both knobs user-overridable.
+    unsafe {
+        if env::var_os("GDK_DEBUG").is_none() {
+            env::set_var("GDK_DEBUG", "gl-prefer-gl");
+        }
+        if env::var_os("GSK_RENDERER").is_none() {
+            env::set_var("GSK_RENDERER", "gl");
+        }
+    }
+
     tracing_subscriber::fmt::init();
 
     let data_dir = dirs::data_dir()