You can't have a drop-down combobox style control if you have a multiline textarea afaik. This would mean Khaled would have to write a pretty complex custom control to support listbox-style dropdown, or he would have to add a dropdown listbox beneath or above the multiline textbox which would become confusing.
If you can think of a *standard* way to have a multiline textbox that retains the recent topic history, that would probably help. I'm not sure it would be such a great idea to mix in all these custom controls into Windows these days.