Hovering over an autocomplete box always selected the last element
rather than the element under the cursor. This is because resultIndex
was bound by reference.
This looks like a bug in FF implementation of "for (let [x, y] of ...)"
-rather than binding "x" and "y" to the scope of the loop, it's
equivalent to "for (var [x, y] of ...)", which causes nasty anomalies
for functions created inside the loop body.