Strange black box appearing in wpf application : <p><a href="https://i.stack.imgur.com/YUiOz.png" rel="noreferrer"><img src="https://i.stack.imgur.com/YUiOz.png" alt="enter image description here"></a></p>

<p>Hi!</p>

<p>I am new in wpf application development. I just create a demo application and I see that the black box appearing in the top of my application. Can anyone tell me how can I remove this ? </p>

<p>I am sorry, I don't even know the name of it.</p>
