verbose debug logging

systems/x86_64-linux/drivebystation-nix/mutter-tearing.nix

@@ -14,6 +14,10 @@ in
       mutter = prev.mutter.overrideAttrs (old: {
         pname = "mutter-tearing";
 
+        # nixpkgs leaves Mutter's verbose topics compiled out by default.
+        # This is required for MUTTER_DEBUG=kms to emit the async commit flags.
+        mesonFlags = (old.mesonFlags or [ ]) ++ [ "-Dverbose=true" ];
+
         # The MR currently targets Mutter 51. Apply it to the packaged Mutter
         # 50 source, then resolve the four small context conflicts separately.
         prePatch = ''
@@ -44,7 +48,12 @@ in
     })
   ];
 
+  environment.sessionVariables = {
     # Mutter keeps tearing support behind an experimental runtime switch.
-  environment.sessionVariables.MUTTER_DEBUG_EXPERIMENTAL_FEATURES =
-    lib.mkDefault "tearing";
+    MUTTER_DEBUG_EXPERIMENTAL_FEATURES = lib.mkDefault "tearing";
+
+    # Temporarily expose whether async atomic commits reach KMS or fall back
+    # to synchronized presentation. Remove this after collecting the log.
+    MUTTER_DEBUG = lib.mkDefault "kms";
+  };
 }