Fix getAttribLocation to return -1 for built-in attributes per WebGL 2.0 spec

[Copilot]

gl.getAttribLocation() was returning 0 for built-in attributes (gl_VertexID, gl_InstanceID, gl_Position, gl_PointSize) instead of the spec-mandated -1, causing Khronos conformance test failures.

Changes

• src/client/graphics/webgl_context.cpp
  • Added early return check for built-in attribute names in WebGLContext::getAttribLocation()
  • Returns nullopt for built-in attributes (converted to -1 by script bindings layer)
  • Added <set> include

// WebGL 2.0 spec §5.18: Built-in attributes always return -1
static const std::set<string> kBuiltInAttribs = {
  "gl_VertexID", "gl_InstanceID", "gl_Position", "gl_PointSize"};
if (kBuiltInAttribs.count(name))
{
  return nullopt;  // Returns nullopt which script bindings converts to -1
}

Architecture

Implementation is in the graphics layer (client/graphics/webgl_context.cpp) rather than script bindings layer, providing better separation of concerns and centralizing WebGL spec compliance logic at the appropriate architectural level.

Spec Reference

WebGL 2.0 §5.18: "Applications may query the location of a built-in attribute by passing its name to getAttribLocation, but this will always return -1."

• Fixes WebGL2: gl.getAttribLocation for built-in attribute 'gl_VertexID' returns incorrect value (should be -1, got 0) #411

Original prompt

---

This section details on the original issue you should resolve

<issue_title>WebGL2: gl.getAttribLocation for built-in attribute 'gl_VertexID' returns incorrect value (should be -1, got 0)</issue_title>
<issue_description>## Bug Description
Running Khronos/WebGL2 conformance test (active-built-in-attribs.html) in JSAR Runtime:

shouldBe('activeInfo.name', '"gl_VertexID"');
attribLoc = gl.getAttribLocation(prog, 'gl_VertexID');
wtu.glErrorShouldBe(gl, gl.NO_ERROR, "Should be able to request the location of a built-in.");
shouldBe('attribLoc', '-1');

Test fails: attribLoc should be -1. Was 0.

Spec Reference

• WebGL 2.0 Spec §5.18 gl.getAttribLocation
  • For built-in attributes (like gl_VertexID, gl_InstanceID, gl_Position, gl_PointSize):

    "Applications may query the location of a built-in attribute by passing its name to getAttribLocation, but this will always return -1."

• MDN
• Chromium Implementation

Root Cause

• JSAR currently returns 0 for built-in attribute names instead of -1, violating spec compliance.
• Correct behavior: gl.getAttribLocation(program, 'gl_VertexID') MUST return -1, never a valid location.

Acceptance Criteria

• getAttribLocation on any built-in attribute ('gl_VertexID', 'gl_InstanceID', 'gl_Position', 'gl_PointSize') MUST always return -1
• The referenced conformance test passes

Implementation Guidance (C++)

• Fix the attribute location query in WebGLRenderingContext/WebGL2RenderingContext
• Check if the attribute name matches any of the built-in names, and always return -1

int WebGLRenderingContext::getAttribLocation(WebGLProgram* program, const std::string& name) {
    static const std::set<std::string> kBuiltInAttribs = {
        "gl_VertexID", "gl_InstanceID", "gl_Position", "gl_PointSize"
    };
    if (kBuiltInAttribs.count(name)) {
        return -1;
    }
    // ... normal lookup logic
}

• Update error handling and unit tests for built-in attribute query

Impact

• Strict compliance with WebGL2 specification
• Enables all Khronos/WebGL2 tests for built-in attrib queries to pass

---

Relevant test HTML:

• https://github.com/EndlessJour9527/WebGL/blob/894d3ba6239ec021a2b65d65d368324c1088aa15/sdk/tests/conformance2/programs/active-built-in-attribs.html
  Spec:
• https://registry.khronos.org/webgl/specs/latest/2.0/#5.18
  Chromium ref:
• https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/renderer/modules/webgl/webgl_rendering_context_base.cc;l=3742
  </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes WebGL2: gl.getAttribLocation for built-in attribute 'gl_VertexID' returns incorrect value (should be -1, got 0) #411

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[EndlessJour9527]

@copilot please impl in client/graphics

[Copilot]

Done! Moved the implementation to client/graphics/webgl_context.cpp in commit c135954. The built-in attribute check now happens at the graphics layer in WebGLContext::getAttribLocation(), which returns nullopt for built-in attributes. The script bindings layer correctly converts this to -1.