What is it on the left-top of my screen?

Dont know when it appears. Just like some kind of small nail sticky on my screen. Cannot move it, nor any menu coming up. I don’t even know what that is.

What I’m pretty sure is it belongs to Unity, because once I clicked on it, the Unity3D Editor became focused.

Could anybody know what that is and tell me how to get rid of it?

Thank you !

I’m pretty sure that is being drawn by other application. Close all other processes but Unity, to see which one was drawing that icon.

You mean the application icon? Does it stay even when you minimize Unity3d?