So, when you click one of the foods in your inventory, the info popup appears on the right of the food, which puts it completely off-screen. Maybe have the info popup appear on the left instead?
There is also a serious issue with getting the popups to appear at all when clicking. Maybe use right-click instead? Nothing else uses right-click, and I kind of reflexively think to use right-click for things like that anyways.
There's also no indication that you can click the clipboard to move it out of the way. Maybe put a little arrow on the edge pointing to the left to show that?
Another thing that'd be nice is something to indicate when a passenger is at max size so you don't waste the fattening food on them. Or just make it so you can't use it on them.