changing a single character causes `runCode` to be called twice

Steps to reproduce:
- write a program the draws a `rect`
- put a breakpoint in `runCode` in live-editor.output.js
- highlight the first char of one of the numbers and change it
- notice that `runCode` fires twice

We should avoid doing this so that we're not wasting CPU/battery.

Start in live-editor.output.js and reproduce the issue by placing a breakpoint in runCode, then changing one selected character in a number from a program that draws a rect. Trace why the edit triggers runCode twice. Done means the same edit invokes runCode only once without changing the resulting program behavior.

Written by the indexing model from the issue text.