R1:962e60c561e8

Retain focused node when redrawing tokenizer/typeahead results

Summary:
Ref T4420. Fixes T5473. Currently, when typeahead results get redrawn, you can lose your cursor position. A simple way to reproduce this is type "dif", select "Differential" using the arrow keys, then type "f". The selection will be lost.

Instead: store the old selection, then look for an item with the same name in the new set and select it. In effect, this preserves any focus selection.

Test Plan:
- Typed "dif".
- Typed "down…