Interesting. I'm not able to reproduce, I'm on a Macbook Pro. I see that you have changed the default theme. It's possible the theme is at fault, or that it is a bug in Brackets.
Muhammad -- my guess is it's due to the old version of Node.js that's being used with Brackets (it's build into the XDK, you cannot replace it). Our next release will include a significant update to the version of node used in the XDK as well as the version used by the Brackets sub-component (they are independent). Interested to see if that release makes a difference.
I'm also on Sierra and just tried again to reproduce this issue and still cannot.
It's possible that it is being caused by a brackets theme or plugin.
One way you might check is to do a complete uninstall:
Then reinstall and see if the problem was fixed.
Of course, you'd lose any customizations that you've made to Intel XDK including additional themes and plugins.
Sorry I don't have any more convenient suggestions!