Progress gauge bug

I've found a way to repeat a bug that has annoyed me with progress gauges for some time. Here's how: Make sure font-lock is turned on. Visit a large file, e.g. a ChangeLog. Then type `C-x i ChangeLog RET', i.e. insert the ChangeLog contents into itself. After this, font-lock and progress gauge seem to lock into an endless loop. If you use C-g to break out of the loop, font-lock-pre-idle-hook reports an error. This is all 100% repeatable, with -vanilla, under X (Athena) and Linux.