WPF Editable Combobox with DropDown list where in traversing thru dropdown items do not affect textbox

Question

I have removed Drop Down button (toggle button) from the combobox template. To make it look like a TextBox with pop-up containing Items.

If I type '10' then all matching data is populated and dropdown is opened with the data, e.g. data is: 10, 100, 1000.

Now, if I press the 'DoWN' arrow then I want to traverse thru the list, 10,100,1000. I do not want to change the text in editable textbox, i.e. It should be always '10', which is the default behaviour of the combobox.

Any help would be highly appreciated ?