Do any of the undocumented options in Slip.js help?

The app should not allow me to add duplicate items to the list. If I try to submit something and there's another item with the exact same text already in the list, it should not submit a new item and clear the text field but instead just show a "snackbar" message at the bottom saying something like "That's already in your list.".

Use a cache-first strategy rather than a network first one to get faster loading times.

Also figure out why the page often isn't refreshing properly on the phone. Current ServiceWorker shouldn't cause it getting stuck with cache entries. Yet still I sometimes can only get it to refresh properly using remote debugging.

Get rid of delay by using touchstart/touchend without interferring with drag&drop.

A white shopping cart with a black border would be ideal.

When adding items in Chrome on Android, sometimes they keyboard will close even though the textfield is still focussed. This seems to be happening when pressing the add button. Not sure if it happens when submitting the form via the keyboard too.

Would be cool to have a way to take all your data and move it to a different device.

At the moment when you have typed in the name of the item and either submitted via the keyboard or pressed the add button, the item appears in the list and the view scrolls to the bottom so you can see it there.
While entering stuff I often didn't notice though that it actually appeared in the list and found myself checking. Maybe a nice, non-obtrusive animation would help make me notice the item was actually successfully added.

At the moment it fits nicely on my phones screen but it actually seems to have a fixed width and on larger resolutions it's too small. Tried adding an .mdl-layout-spacer between the text field and the button but that just pushed the button slightly out of view in some browsers.

Every action on a list creates a previous state which captures the whole state of the list before the action so that the action can be undone.
To make sure memory isn't poluted endlessly the previous states should be capped at a reasonable number. When adding a previous state and the total number of previous states is larger than that number, the oldest ones should be removed until they're below that number again.

Probably using webpack.

When typing to add a new item there should be a way to receive suggestions from previously entered items, to make adding items you buy often faster.
This could either be a list that pops up over the text field (which might be in the way though) or it could be something you explicitly pick from rather than typing in values manually (but then you'd probably still want to find stuff).