// Ensure exceeding the high watermark makes channel unwritable.
        ch.write(buffer().writeZero(128));
        assertThat(buf.toString(), is("false "));
        // Ensure going down to the low watermark makes channel writable again by flushing the first write.
        assertThat(ch.unsafe().outboundBuffer().remove(), is(true));
        assertThat(ch.unsafe().outboundBuffer().totalPendingWriteBytes(), is(128L));
        assertThat(buf.toString(), is("false true "));
        safeClose(ch);
    }