When you eat a donut, cheese or anything else that heals over time you can't open your inventory for the time that you're healing + some additional seconds (a total of 20-30seconds i'd say). If you keep ur inventory open after u eat food, it'll stay open (without being able to close it) for those 20-30seconds, blocking your visual and making it impossible to PK since your inventory is open in front of ur character.
I see no reason for food to act like this..fill me in if u think there actually is a reason.