Enable CI Rendering on Mac (#2673)

## Change

- Turn on headless rendering on Mac for MaterialXView
- Added to the MacOS_Xcode_16_Python313 workflow as that already does shader validation testing.

Notes:

- MaterialxGraphEditor rendering not enabled as it's using OpenGL which has issues running headless. (A[ new issue](#2672) has been logged to support Metal rendering here. If / when it is available then it can be turned on)
- The Metal environment set up here is the same pre-requisite as for this [PR for full test suite enabling](#2675).

Author: kwokcb

.github/workflows/main.yml

@@ -78,6 +78,7 @@ jobs:
           compiler_version: "16.4"
           python: 3.13
           test_shaders: ON
+          test_render: ON
 
         - name: MacOS_Xcode_26_Python313
           os: macos-26
@@ -265,25 +266,40 @@ jobs:
         fi
         cppcheck --project=build/compile_commands.json --error-exitcode=1 --suppress=normalCheckLevelMaxBranches --suppress=*:*/External/* --suppress=*:*/NanoGUI/*
 
-    - name: Initialize Virtual Framebuffer
+    - name: Setup Rendering Environment (Linux)
       if: matrix.test_render == 'ON' && runner.os == 'Linux'
       run: |
         Xvfb :1 -screen 0 1280x960x24 &
         echo "DISPLAY=:1" >> $GITHUB_ENV
         echo "LIBGL_ALWAYS_SOFTWARE=1" >> $GITHUB_ENV
 
+    - name: Setup Rendering Environment (MacOS)
+      if: matrix.test_render == 'ON' && runner.os == 'macOS'
+      run: |
+        # macOS can render headless with Metal backend without virtual display
+        # Force software rendering for Metal backend (more reliable in CI)
+        echo "MTL_HARDWARE_RENDERING=0" >> $GITHUB_ENV
+        # Enable Metal debug layer for better debugging in CI
+        echo "MTL_DEBUG_LAYER=1" >> $GITHUB_ENV
+        echo "LIBGL_ALWAYS_SOFTWARE=1" >> $GITHUB_ENV
+
     - name: Render Script Tests
       if: matrix.test_render == 'ON'
       run: |
         mkdir build/render
         python python/Scripts/baketextures.py resources/Materials/Examples/StandardSurface/standard_surface_brass_tiled.mtlx build/render/brass_average_baked.mtlx --average
         python python/Scripts/translateshader.py resources/Materials/Examples/StandardSurface/standard_surface_carpaint.mtlx build/render/usd_preview_surface_carpaint.mtlx UsdPreviewSurface --hdr
 
-    - name: Render Application Tests
+    - name: Viewer Tests 
       if: matrix.test_render == 'ON'
       run: |
         ../installed/bin/MaterialXView --material brass_average_baked.mtlx --mesh ../../resources/Geometry/sphere.obj --screenWidth 128 --screenHeight 128 --cameraZoom 1.4 --shadowMap false --captureFilename Viewer_BrassAverage.png
         ../installed/bin/MaterialXView --material usd_preview_surface_carpaint.mtlx --mesh ../../resources/Geometry/sphere.obj --screenWidth 128 --screenHeight 128 --cameraZoom 1.4 --shadowMap false --captureFilename Viewer_CarpaintTranslated.png
+      working-directory: build/render
+
+    - name: Graph Editor Tests
+      if: matrix.test_render == 'ON' && runner.os == 'Linux'
+      run: |
         ../installed/bin/MaterialXGraphEditor --material ../../resources/Materials/Examples/StandardSurface/standard_surface_marble_solid.mtlx --viewWidth 128 --viewHeight 128 --captureFilename GraphEditor_MarbleSolid.png
       working-directory: build/render
 

source/MaterialXGraphEditor/Main.cpp

@@ -167,6 +167,13 @@ int main(int argc, char* const argv[])
     glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR, 0);
 #endif
 
+    // When captureFilename is specified, make window invisible
+    // to avoid issues with headless rendering.
+    if (!captureFilename.empty())
+    {
+        glfwWindowHint(GLFW_VISIBLE, GL_FALSE);
+    }
+
     // Create window with graphics context
     GLFWwindow* window = glfwCreateWindow(1280, 960, "MaterialX Graph Editor", NULL, NULL);
     if (!window)