Background

When running WebGL2 code or Khronos conformance tests in JSAR Runtime, the following error is encountered:

Uncaught Error: Failed to execute 'transformFeedbackVaryings' on 'WebGL2RenderingContext': Not implemented

This occurs when JS/WebGL2 code calls the API:

gl.transformFeedbackVaryings(program, ['gl_Position'], gl.INTERLEAVED_ATTRIBS);

Impact

• Standard conformance tests requiring transform feedback fail to run in JSAR
• Popular libraries that use transform feedback (e.g., Three.js advanced rendering patterns, Babylon.js particle systems, GPU-based computation, etc.) fail or degrade
• Both classic and XR spatial apps depending on this API are blocked

Web Standard Reference

• WebGL 2.0 Specification §5.19.3 transformFeedbackVaryings
• Chromium implementation (WebGL2RenderingContextBase::TransformFeedbackVaryings)
• MDN docs - transformFeedbackVaryings

Root Cause

• The native C++ binding for WebGL2RenderingContext.transformFeedbackVaryings is missing or stubbed as "not implemented" in JSAR
• No logic dispatching from JS layer to the actual OpenGL call (glTransformFeedbackVaryings)

Acceptance Criteria

• Calling gl.transformFeedbackVaryings in JSAR WebGL2 context correctly dispatches to native and configures transform feedback as per spec
• Conformance tests referencing this API pass
• Popular libraries relying on transform feedback work (Three.js, Babylon.js examples)

Implementation Guidance (C++)

• Add JS binding for transformFeedbackVaryings in WebGL2RenderingContext
• Dispatch to corresponding OpenGL API:

void WebGL2RenderingContext::transformFeedbackVaryings(WebGLProgram* program, const std::vector<std::string>& varyings, GLenum bufferMode) {
    // Convert string array to C array
    std::vector<const char*> c_varyings;
    for (const auto& v : varyings) {
        c_varyings.push_back(v.c_str());
    }
    glTransformFeedbackVaryings(program->glObject(), c_varyings.size(), c_varyings.data(), bufferMode);
}

• Update JS-to-native binding so the method is available on WebGL2RenderingContext
• Fails gracefully (throws per spec) if invoked on invalid program object

Relevant code location to update:

• src/client/builtin_scene/webgl_rendering_context.cpp

Supporting this API is required for WebGL2 spec conformance and ecosystem compatibility.