Opening the inventory or a chest show the helmet description/the top left corner item regardless of where your mouse is.

When you open your inventory in minecraft windows edition, your mouse appear at the last place you left the last time you opened your inventory.

But for some reasons, it show the description of what you have on your head, even when you left your mouse outside of the menu.

EDIT :

I just found out that it does the same with all the blocks that can contain something, like chests, hoppers, but in these cases it show the top left item's description.