I guess you are using a codemirror line widget which when you display the widget, pops all lines down when displaying it.
I think I'd prefer the way the autocompleter dropdown works in LT and how I have implemented it in elm-light where the popup is displayed over the code (but positioned under the marked error of course).
Might be a bit of hassle to implement, but hopefully possible even though you are using the codemirror plugin ?
See images:


Maybe with some transparency for good show too ! :-)