Do we need both showAllItems and trigger here? Could trigger === 'manual' be used instead to determine whether to show all items?

I figureed I would include both so that consumers of useComboBoxState wouldn't be forced into the "manual or focus trigger = show all items" logic, felt like this gives them more freedom to define what would get displayed and when for their own ComboBox implementations.

hmm not sure I follow. you mean if someone called comboBoxState.open() manually?

yeah, or if someone wrote their own useComboBox but used our useComboBoxState and wanted their combobox not to show all items when opening the menu. They could then have something like this in their custom useComboBox hook:

let onKeyDown = (e: KeyboardEvent) => {
    switch (e.key) {
      case 'ArrowDown':
        state.open('first', false, 'manual');

meaning their default combobox behavior would be to only show the filtered collection at all times

Hmm, I think this may be providing too much flexibility. Seems like in that case maybe it would be preferable to add a prop to control the behavior for everyone equally. I guess if we decide to do that it would also solve this. For now, I think keeping the API simple is preferable.

Why is it a "tracker"?

Different name here because the open and toggle functions have showAllItems as a param. Maybe I could rename this to displayAllItems? I definitely could change this to showAllItems if I remove that parameter, but that's subject to the discussion in a previous comment

Seems a little hacky to use a ref for this... I guess the other option would be to not use menu trigger state at all and reimplement it and that seems possibly worse 🤔

yeah def a bit hacky, but as you said the alternative would be to reimplement the open state logic from useMenuTriggerState+useOverlayTriggerState just so we can get onOpenChange to return the menu trigger action as well. I figured changing those hooks themselves to track menu trigger action was undesirable as well.