I need exactly same feature as following dropdown or combo
Typable inside combo and auto complete,
2000 of items or more.
Should not show items list while typing.
Should show items list after pressing downkey or button.
while showing items list, must shows 3 or 4 items at a time and scoll and focus.
Hope you get my problem.
Take a look at jQuery Autocomplete. It has most most features you require, and you can probably customize it to get the rest.
At least you get a solid basis to build upon.