diff --git a/test/io/console/test_io_console.rb b/test/io/console/test_io_console.rb
index 1aff99aa03..165012d0d7 100644
--- a/test/io/console/test_io_console.rb
+++ b/test/io/console/test_io_console.rb
@@ -11,7 +11,17 @@ class TestIO_Console < Test::Unit::TestCase
   PATHS.uniq!
 
   # FreeBSD seems to hang on TTOU when running parallel tests
-  # tested on FreeBSD 11.x
+  # tested on FreeBSD 11.x.
+  #
+  # Solaris gets stuck too, even in non-parallel mode.
+  # It occurs only in chkbuild.  It does not occur when running
+  # `make test-all` in SSH terminal.
+  #
+  # I suspect that it occurs only when having no TTY.
+  # (Parallel mode runs tests in child processes, so I guess
+  # they has no TTY.)
+  # But it does not occur in `make test-all > /dev/null`, so
+  # there should be an additional factor, I guess.
   def set_winsize_setup
     @old_ttou = trap(:TTOU, 'IGNORE') if RUBY_PLATFORM =~ /freebsd|solaris/i
   end