Stage3D: Render vertex/index buffer partially issue

Question

I've tried to call  Context3D:drawTriangles for part of vertex/index buffers and nothing is rendered.
This can be reproduced with HelloTriangle example from here: http://www.adobe.com/devnet/flashplayer/articles/hello-triangle.html
It works fine if you allocated buffers for 3 indices and 3 vertices. But if I change
context3D.drawTriangles(indexbuffer) to context3D.drawTriangles(indexbuffer, 0, 1)
and allocate one more item either in vertex or index buffer - nothing is rendered.
Is it a bug or am I missing something?
My changes in code:
context3D.drawTriangles(indexbuffer, 0, 1);
and
vertexbuffer = context3D.createVertexBuffer(3 + 1, 6);
or
indexbuffer = context3D.createIndexBuffer(3 + 1);
Below is complete code from sample with my changes:
package {
import com.adobe.utils.AGALMiniAssembler;
import flash.display.Sprite;
import flash.display3D.Context3D;
import flash.display3D.Context3DProgramType;
import flash.display3D.Context3DVertexBufferFormat;
import flash.display3D.IndexBuffer3D;
import flash.display3D.Program3D;
import flash.display3D.VertexBuffer3D;
import flash.events.Event;
import flash.geom.Matrix3D;
import flash.geom.Rectangle;
import flash.geom.Vector3D;
import flash.utils.getTimer;
[SWF(width="800", height="600", frameRate="60", backgroundColor="#FFFFFF")]
public class VertexBufferTest extends Sprite
    protected var context3D:Context3D;
    protected var program:Program3D;
    protected var vertexbuffer:VertexBuffer3D;
    protected var indexbuffer:IndexBuffer3D;
    public function VertexBufferTest()
        stage.stage3Ds[0].addEventListener( Event.CONTEXT3D_CREATE, initMolehill );
        stage.stage3Ds[0].requestContext3D();
        addEventListener(Event.ENTER_FRAME, onRender);
    protected function initMolehill(e:Event):void
        context3D = stage.stage3Ds[0].context3D;
        context3D.configureBackBuffer(800, 600, 1, false);
        var vertices:Vector.<Number> = Vector.<Number>([
            -0.3,-0.3,0, 1, 0, 0, // x, y, z, r, g, b
            -0.3, 0.3, 0, 0, 1, 0,
            0.3, 0.3, 0, 0, 0, 1]);
        // Create VertexBuffer3D. 3 vertices, of 6 Numbers each
        vertexbuffer = context3D.createVertexBuffer(3, 6);
        // Upload VertexBuffer3D to GPU. Offset 0, 3 vertices
        vertexbuffer.uploadFromVector(vertices, 0, 3);
        var indices:Vector.<uint> = Vector.<uint>([0, 1, 2]);
        // Create IndexBuffer3D. Total of 3 indices. 1 triangle of 3 vertices
        indexbuffer = context3D.createIndexBuffer(3 + 1);
        // Upload IndexBuffer3D to GPU. Offset 0, count 3
        indexbuffer.uploadFromVector (indices, 0, 3);
        var vertexShaderAssembler : AGALMiniAssembler = new AGALMiniAssembler();
        vertexShaderAssembler.assemble( Context3DProgramType.VERTEX,
                "m44 op, va0, vc0\n" + // pos to clipspace
                        "mov v0, va1" // copy color
        var fragmentShaderAssembler : AGALMiniAssembler= new AGALMiniAssembler();
        fragmentShaderAssembler.assemble( Context3DProgramType.FRAGMENT,
                "mov oc, v0"
        program = context3D.createProgram();
        program.upload( vertexShaderAssembler.agalcode, fragmentShaderAssembler.agalcode);
    protected function onRender(e:Event):void
        if ( !context3D )
            return;
        context3D.clear ( 1, 1, 1, 1 );
        // vertex position to attribute register 0
        context3D.setVertexBufferAt (0, vertexbuffer, 0, Context3DVertexBufferFormat.FLOAT_3);
        // color to attribute register 1
        context3D.setVertexBufferAt(1, vertexbuffer, 3, Context3DVertexBufferFormat.FLOAT_3);
        // assign shader program
        context3D.setProgram(program);
        var m:Matrix3D = new Matrix3D();
        m.appendRotation(getTimer()/40, Vector3D.Z_AXIS);
        context3D.setProgramConstantsFromMatrix(Context3DProgramType.VERTEX, 0, m, true);
        context3D.drawTriangles(indexbuffer, 0, 1);
        context3D.present();

Answer 1

I've tried to call  Context3D:drawTriangles for part of vertex/index buffers and nothing is rendered.
This can be reproduced with HelloTriangle example from here: http://www.adobe.com/devnet/flashplayer/articles/hello-triangle.html
It works fine if you allocated buffers for 3 indices and 3 vertices. But if I change
context3D.drawTriangles(indexbuffer) to context3D.drawTriangles(indexbuffer, 0, 1)
and allocate one more item either in vertex or index buffer - nothing is rendered.
Is it a bug or am I missing something?
My changes in code:
context3D.drawTriangles(indexbuffer, 0, 1);
and
vertexbuffer = context3D.createVertexBuffer(3 + 1, 6);
or
indexbuffer = context3D.createIndexBuffer(3 + 1);
Below is complete code from sample with my changes:
package {
import com.adobe.utils.AGALMiniAssembler;
import flash.display.Sprite;
import flash.display3D.Context3D;
import flash.display3D.Context3DProgramType;
import flash.display3D.Context3DVertexBufferFormat;
import flash.display3D.IndexBuffer3D;
import flash.display3D.Program3D;
import flash.display3D.VertexBuffer3D;
import flash.events.Event;
import flash.geom.Matrix3D;
import flash.geom.Rectangle;
import flash.geom.Vector3D;
import flash.utils.getTimer;
[SWF(width="800", height="600", frameRate="60", backgroundColor="#FFFFFF")]
public class VertexBufferTest extends Sprite
    protected var context3D:Context3D;
    protected var program:Program3D;
    protected var vertexbuffer:VertexBuffer3D;
    protected var indexbuffer:IndexBuffer3D;
    public function VertexBufferTest()
        stage.stage3Ds[0].addEventListener( Event.CONTEXT3D_CREATE, initMolehill );
        stage.stage3Ds[0].requestContext3D();
        addEventListener(Event.ENTER_FRAME, onRender);
    protected function initMolehill(e:Event):void
        context3D = stage.stage3Ds[0].context3D;
        context3D.configureBackBuffer(800, 600, 1, false);
        var vertices:Vector.<Number> = Vector.<Number>([
            -0.3,-0.3,0, 1, 0, 0, // x, y, z, r, g, b
            -0.3, 0.3, 0, 0, 1, 0,
            0.3, 0.3, 0, 0, 0, 1]);
        // Create VertexBuffer3D. 3 vertices, of 6 Numbers each
        vertexbuffer = context3D.createVertexBuffer(3, 6);
        // Upload VertexBuffer3D to GPU. Offset 0, 3 vertices
        vertexbuffer.uploadFromVector(vertices, 0, 3);
        var indices:Vector.<uint> = Vector.<uint>([0, 1, 2]);
        // Create IndexBuffer3D. Total of 3 indices. 1 triangle of 3 vertices
        indexbuffer = context3D.createIndexBuffer(3 + 1);
        // Upload IndexBuffer3D to GPU. Offset 0, count 3
        indexbuffer.uploadFromVector (indices, 0, 3);
        var vertexShaderAssembler : AGALMiniAssembler = new AGALMiniAssembler();
        vertexShaderAssembler.assemble( Context3DProgramType.VERTEX,
                "m44 op, va0, vc0\n" + // pos to clipspace
                        "mov v0, va1" // copy color
        var fragmentShaderAssembler : AGALMiniAssembler= new AGALMiniAssembler();
        fragmentShaderAssembler.assemble( Context3DProgramType.FRAGMENT,
                "mov oc, v0"
        program = context3D.createProgram();
        program.upload( vertexShaderAssembler.agalcode, fragmentShaderAssembler.agalcode);
    protected function onRender(e:Event):void
        if ( !context3D )
            return;
        context3D.clear ( 1, 1, 1, 1 );
        // vertex position to attribute register 0
        context3D.setVertexBufferAt (0, vertexbuffer, 0, Context3DVertexBufferFormat.FLOAT_3);
        // color to attribute register 1
        context3D.setVertexBufferAt(1, vertexbuffer, 3, Context3DVertexBufferFormat.FLOAT_3);
        // assign shader program
        context3D.setProgram(program);
        var m:Matrix3D = new Matrix3D();
        m.appendRotation(getTimer()/40, Vector3D.Z_AXIS);
        context3D.setProgramConstantsFromMatrix(Context3DProgramType.VERTEX, 0, m, true);
        context3D.drawTriangles(indexbuffer, 0, 1);
        context3D.present();