lets start with my use case:
i planned on having buffer chests across a huge rail network that is fully roboported.
furthermore the rail network's power poles carry both red & green circuit wires.
these buffer chests should set their contents to whatever signal i send to them.
now the problem is this: when you remove/lower a filter the chest still has its stock.

what this mod does:
any item that is not in (or more than) the logistic filter set for it gets poured onto the ground.
example A: chest has a filter set for 200 rails, contains 200 rails, and you lower it to 100, then it dumps 100 railes.
example B: you add items to it that do not have a filter set at all, all of them get dumped onto the ground.
example C: both of these situations combined. ^

why it dumps onto the ground:
in the lua api i wasn't able to find a function that asks the logistic network to "please pickup X from buffer chest Y".
it appears that the only way to do that would be to temporary replace the chest with a pink one to trigger the pickup.
so instead of an ugly swaparoo i decided to just dump the items on the ground and mark them for deconstruction,
that way bots will still have to travel "to it" to retrieve the items, its not an elegant solution but i can live with it.

whats next:
i'll probably pitch this on the factorio reddit in the near feature with the following key points;
- turn the [ ] slider on buffer chests into a single < > slider to visually mimic player request/trash slots (same number)
- add a checkbox to the buffer chests (like how blue can request from green) roughly like "return overstock to network"
- instead of dumping it onto the ground like this polyfill mod, have it request the items via logistic bots
- (or in other words: add trash slots to buffer chests, with an option to trash unfiltered items as well)
- ^ done: https://www.reddit.com/r/factorio/comments/gwg1j5/buffer_overstock/

(note that the mod thumbnail is just a visual representation of the concept, in game the buffer chests look normal)