What is happening:
The sunglasses that are received from Eagle Eye are considered a quest item, and quest items are unequippable (at least in the Linux version).
What should be happening:
Either the sunglasses should not be a quest item, or quest items should be able to be equipped.
Steps to reproduce the issue:
Receive the sunglasses after a successful completion of "Eye Sore."
The item can be dragged to the regular inventory, but it is still unequippable.
On the Please Mr. Post Pony you also get a cookie as a reward. The cookie also ends on the Quest Items slots but you can drag it to the regular inventory and it is usable (right click on the Cookie and then click on use).
Also I noticed that the mail bag item from the quest remained with the name Cloudopolis Post Office Mail, and on the description it says "Please make sure that the mail is properly delivered"
It seems it doesn't unlocks so you can use it as a saddle bag.
What is the purpose of dragging into the inventory, I have another problem that was mentioned here retro bowl