我什么时候应该在 WebGL/OpenGL 中启用/禁用顶点位置属性？

var vsThatUses2Attributes = `
  attribute vec4 position;
  attribute vec2 texcoord;
  
  varying vec2 v_texcoord;
  
  void main() {
    v_texcoord = texcoord;
    gl_Position = position;
  }
`;

var vsThatUses1Attribute = `
  attribute vec4 position;
  
  varying vec2 v_texcoord;
  
  void main() {
    v_texcoord = position.xy * 0.5 + 0.5;
    gl_Position = position + vec4(1, 0, 0, 0);
  }
`;

var fs = `
  precision mediump float;
  varying vec2 v_texcoord;
  
  void main () {
    gl_FragColor = vec4(v_texcoord, v_texcoord.x * v_texcoord.y, 1);
  }
`;

var gl = document.querySelector("canvas").getContext("webgl");
document.body.appendChild(gl.canvas);
var programThatUses2Attributes = twgl.createProgramFromSources(gl, [vsThatUses2Attributes, fs]);
var programThatUses1Attribute = twgl.createProgramFromSources(gl, [vsThatUses1Attribute, fs]);

var positionLocation2AttribProg = gl.getAttribLocation(programThatUses2Attributes, "position");
var texcoordLocation2AttribProg = gl.getAttribLocation(programThatUses2Attributes, "texcoord");

var positionLocation1AttribProg = gl.getAttribLocation(programThatUses1Attribute, "position");

var positionBufferFor2AttribPrg = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBufferFor2AttribPrg);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
  -1, -1,
 -.5,  1,
   0, -1,
]), gl.STATIC_DRAW);

var texcoordBufferFor2AttribPrg = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBufferFor2AttribPrg);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
  0, 0,
0.5, 1,
  1, 0,
]), gl.STATIC_DRAW);


var positionBufferFor1AttribPrg = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBufferFor1AttribPrg);
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array([
  -1, -1,
   0, -1,
  -1,  1,
  -1,  1,
   0, -1,
   0,  1,
]), gl.STATIC_DRAW);


// turn on 2 attributes
gl.enableVertexAttribArray(positionLocation2AttribProg);
gl.enableVertexAttribArray(texcoordLocation2AttribProg);

gl.bindBuffer(gl.ARRAY_BUFFER, positionBufferFor2AttribPrg);
gl.vertexAttribPointer(positionLocation2AttribProg, 2, gl.FLOAT, false, 0, 0);
gl.bindBuffer(gl.ARRAY_BUFFER, texcoordBufferFor2AttribPrg);
gl.vertexAttribPointer(texcoordLocation2AttribProg, 2, gl.FLOAT, false, 0, 0);

// draw with 2 attributes enabled
gl.useProgram(programThatUses2Attributes);
gl.drawArrays(gl.TRIANGLES, 0, 3);

// setup for second program that uses only 1 attribute
gl.bindBuffer(gl.ARRAY_BUFFER, positionBufferFor1AttribPrg);
gl.vertexAttribPointer(positionLocation1AttribProg, 2, gl.FLOAT, false, 0, 0);

// NOTICE WE HAVE !NOT turned off other attribute
gl.useProgram(programThatUses1Attribute);
gl.drawArrays(gl.TRIANGLES, 0, 6);

log("glError:", gl.getError());

function log() {
  var pre = document.createElement("pre");
  pre.appendChild(document.createTextNode(Array.prototype.join.call(arguments, " ")));
  document.body.appendChild(pre);
}

canvas { border: 1px solid black; }

<script src="https://twgljs.org/dist/twgl.min.js"></script>
<pre>
This example draws a triangle with 3 vertices using 2 attributes.

It then draws a quad using 6 vertices and 1 attribute 
<b>WITHOUT TURNING OFF THE NOW 2nd UNUSED ATTRIBUTE</b>.

That means not only is that attribute left on but it only has 3 vertices even 
though the draw will use 6 vertices. Because that attribute is not 'comsumed' by 
the current program it's ok according to the spec.
</pre>
<canvas width="150" height="30"></canvas>