Under Netscape's nsapi routines, you can detect a failure in net_write(), which includes the user pressing the stop button.

It would appear that there is an Apache equivalent, but you would need to be running something like mod_perl to gain access to the Apache "request write" functions.

From perusing the mod_perl lists, it appears there was a change starting with Apache 1.3.6. So...

• For pre-1.3.6 Apache: You have to handle SIGPIPE
• Apache 1.3.6 and later: $r->print() returns false on write failures