This is a simple mod that adds a little X button to the top right corner of inventories that can be clicked to close that inventory as an alternative to pressing E or Escape. It is a feature in the Windows 10 edition of the game. This mod is client side, so it works even if it is not installed on the server you are playing on.
Here is an example:
Yes, you may put this in your non-profit mod pack.
No, you may not redistribute this for money.
For more information, see the license.
Hope you enjoy!