Error highlight sometimes disappears after a period of time

[gfbee]

It's a bit non-deterministic, but I can get it fairly reliably on Mac OS with: ISL previous chosen, launch DrRacket

Welcome to DrRacket, version 7.0 [3m].
Language: Intermediate Student; memory limit: 2000 MB.

click in the Definitions and type (=), then either click Run or type Command-R and wait while doing nothing for twenty seconds.