Custom Made inputTextBox is flickering

Question

I'm having difficulties creating an inputbox with XNA 4.0.

I can already draw and input text into the inputTextbox, but when I type I have two problems occuring.

1. When I hold down a key I'd suppose it will write that character a lot of times, yet it doesn't. And i don't know why it doesn't.
2. When i press a key the screen flickers, how do i solve that? I've already tried by flipping the backbuffer or something like that too bad it wasn't the solution

Here is the code which belongs to the textbox:

    public TextboxInput(GraphicsDevice graphicsDevice, int width, SpriteFont font)
    {
        this.font = font;
        var fontMeasurements = font.MeasureString("dfgjlJL");
        var height = (int) fontMeasurements.Y;
        renderTarget = new RenderTarget2D(graphicsDevice, width, height);
        Text = new StringBuilder();
        this.graphicsDevice = graphicsDevice;
        spriteBatch = new SpriteBatch(graphicsDevice);
    }

    public void Update(GameTime gameTime)
    {
        if (!HasFocus)
        {
            return;
        }
        var keyboard = Keyboard.GetState();
        foreach (var key in keyboard.GetPressedKeys())
        {
            if (!lastKeyboard.IsKeyUp(key))
            {
                continue;
            }
            if (key == Keys.Delete ||
                key == Keys.Back)
            {
                if (Text.Length == 0)
                {
                    continue;
                }
                Text.Length--;
                renderIsDirty = true;
                continue;
            }
            char character;
            if (!characterByKey.TryGetValue(key, out character))
            {
                continue;
            }
            if (keyboard.IsKeyDown(Keys.LeftShift) ||
                keyboard.IsKeyDown(Keys.RightShift))
            {
                character = Char.ToUpper(character);
            }
            Text.Append(character);
            renderIsDirty = true;
        }

        lastKeyboard = keyboard;

    }

    public void PreDraw()
    {
        if (!renderIsDirty)
        {
            return;
        }
        graphicsDevice.Clear(BackgroundColor);

        renderIsDirty = false;
        graphicsDevice.SetRenderTarget(renderTarget);
        spriteBatch.Begin();
        spriteBatch.DrawString(font, Text, Vector2.Zero, ForegroundColor);
        spriteBatch.End();
        graphicsDevice.SetRenderTargets(null);
    }

    public void Draw()
    {
        spriteBatch.Begin();
        spriteBatch.Draw(renderTarget, Position, Color.White);
        spriteBatch.End();
    }

Answer 1

For the flicker, I think that you should be calling graphicsDevice.Clear(BackgroundColor) in your draw method.

The following code is the reason you don't get more characters when holding down. It is saying only register the key press if the keys last state was up.

if (!lastKeyboard.IsKeyUp(key))
{
    continue;
}