I think the text that is selected and sent by button 3 is hard-coded in acme, so the square brackets acts as delimiters. If you click on the text to the left of the brackets, the plumber will not see the brackets or what's inside them. If you sweep and select yourself, the whole selection goes to the plumber.
Well, I can see this is getting called:
look.c:187: if(m->ndata<messagesize-1024 &&
plumbsendtofid(plumbsendfid, m) >= 0){
and the m->data is the full line, including the trailing
".java:[<lineno>,<colno>]" data.
So it's certainly appears to be sending the data to the plumber.
But I don't get the same behavior from acme as when I send the
same text I see in m->data to plumb(1) directly on the command
line.
Jim