Adding emoji via macOS picker sometimes makes cursor jump or duplicate lines

### Expected
When adding an emoji from the macOS picker, the emoji is added and the cursor moves after it.

### Observed
The results I'm getting are not-100% reproducible, but what I'm experiencing is always (?) after adding at least one emoji


### Reproduced

1. On macOS, open a note that already has multiple lines of content
2. Add an emoji from the emoji picker Ctrl Cmd Space
3. Continue adding emojis. You should eventually experience something similar to my GIF

### Where did you see the bug

- System Make: Apple
- System Model: MacBook Pro 16" Late 2019
- OS: macOS 10.15.7
- Simplenote app version: 2.5.0-beta1

Start by reproducing the issue in Simplenote on macOS 10.15.7 using the Ctrl-Cmd-Space emoji picker and a note with multiple lines. Trace the emoji insertion and cursor movement behavior, then verify that repeated emoji insertion neither moves the cursor unexpectedly nor duplicates lines.

Written by the indexing model from the issue text.