I'm working with the MultiSelect dropdown.
My issue is that feeding my own search control into the header of the dropdown itself seems to be impossible.
Our design calls for this component, but requires a search input within the dropdown itself. I've tried to insert one as a header, but your control is built in a way that forces the dropdown to close when the header of the dropdown is clicked. Making my attempted customization of the component impossible. From quick testing it seems the other dropdowns are built this way as well. Making components work in the header and footer (without closing the popup) would greatly increase the customizability of the dropdowns.
It also seems that the current search input within the button can't be removed or changed to readonly. Our design doesn't expect customizable input in the button. It seems that even if you have filtering turned off it still accepts input that does nothing.