Syntax error shouldn't clear display

Currently, a syntax error will wipe out the display, so typing a new line of code often wipes out the current state.
- [ ] Just leave the display untouched when a syntax error occurs.
- [ ] Display a red highlight or something where the syntax error occurs?

The issue names no files, tests, or entry points. Reproduce a syntax error while code is displayed, then trace the error-handling path; done means the existing display remains unchanged after the error, with any error highlighting treated as a separate requirement.

Written by the indexing model from the issue text.