I did try searching for a common, easily understood idiom which would convey the OP's request. Namely, the phenomenon when just before leaving your house, a guest will suddenly remember an important detail which acts as a trigger for other questions and observations.
It needn't be a guest leaving your house, it could be the other way round.
I used to have a very dear aunt who lived alone with her dog, out in the sticks, in a tiny tiny Italian village. Once after a long visit, she followed my ex-husband and myself to the car. She was talking so I rolled down the car window. Craftily, she leaned in and began chatting away to us. I could tell my ex was getting exasperated. Unperturbed, or knowing full well days may pass before she had the opportunity to speak to another human being for longer than 5 minutes; my aunt hung onto the car window. My ex-husband switched on the engine, and began to slowly move forward, my aunt, still talking, likewise moved along. "Zia!" I cried, "we really have to go now! Ciaooo."
Which lead me to think about my aunt hanging on to every word, clinging onto every word because she didn't want to say good-bye. But its true idiomatic meaning is quite different; to concentrate or pay great attention to every word said by an individual you admire or greatly respect.
I then tried "the long good-bye" but that answer had already been suggested by Mr Hen. A similar phrase which would carry a similar meaning might be, dragging out goodbyes. Google gives 741 results, which isn't too bad and on top of that, it's self-explanatory.
My third suggestion is a neologism, be glued to a door knob
describing that precise moment when a guest's hand is stuck on the door knob.
For half an hour, she hung on to every word she said. She didn't want us to leave.
She dragged out her goodbye for thirty minutes before leaving.
She was glued to the door knob for thirty minutes, I thought she'd never leave.