Hi,

I had the same thing a few times already, but not anymore since the bug was supposed to be fixed a few patches ago.
What I was usually doing for closing this windows was :

1. Move a few meters away from the merchant Mohabi
2. speak to him again and buy one item, any type, it doesn't matter.
3. Move a few meters away again from the merchant AND from the storage chest
4. go on the chest again and open it (even if it is still opened on screen)
5. Right click on the item you bought a few seconds ago to move it from your inventory to the chest
6. Try to close the stash window with the cross on top right corner.

It worked for me and some friends also almost everytime, but as I said, it was 1 week ago and some updates / fixes ago.