[racket] perpetual "Online expansion running"

What does it mean if DrRacket's online Check Syntax indicator is blue, 
and the tooltip says "Online expansion running", but CPU is 0%.

I can run the code.  I can leave DrRacket idle for a long time, but the 
indicator stays blue.  Disabling and then re-enabling online Check 
Syntax doesn't help.  Quitting and restarting DrRacket doesn't help.

A wild guess is that Check Syntax is secretly giving up, due to the 
"require"s in this module (the size of the "require"-d stuff, or the 
complexity of the "require" graph).  There's not much crazy syntax 
transforming happening in the application code, if that matters.

Neil V.