Enabling background expansion does not always immediately trigger background expansion

When background expansion is disabled, enabling it (with the red circle in the lower-right corner of the DrRacket window) sometimes causes the background expansion to start (one has to wait a few seconds and the arrows and blue boxes then appear).

However, this is not always the case. Sometimes enabling background expansion just makes the red circle disappear, and one has to modify one character in the file to trigger background expansion (say add a single space).

Reproduce the issue in the DrRacket window by toggling background expansion with the red circle in the lower-right corner, then compare it with modifying one character in the file. Trace the background-expansion handling from that control; done means enabling it reliably starts expansion without requiring a file edit.