Start a dev build with a fresh user dir. Create
two Ant script, named (using @name in <project>)
e.g. "a" and "b". Run "a" (e.g. click Execute on
it). The tab "a" is created with the output from
"a". Now run "b". The tab "b" is created with the
output from "b", but it is not fronted; "a" is
still the selected output tab. Run "b" again and
it gets fronted.

Generally, when creating a new tab, it is not
automatically put in front of existing tabs. But
if the tab already existed, then it is fronted.

In the Ant module there is:

io = IOProvider.getDefault().getIO(name, false);
io.getOut().reset();
io.select();
// ...
io.setFocusTaken(true);
io.setErrVisible(false);
io.setErrSeparated(false);
new PrintStream(...(io.getErr())).println(...);

which I believe is correct.

It used to work in 3.5 - tabs were fronted.