IDLE restarts when one debugs code raising SystemExit

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-05-04.12:49:03.499>
labels = ['expert-IDLE', 'type-bug', '3.9']
title = 'IDLE restarts when one debugs code raising SystemExit'
updated_at = <Date 2019-09-20.20:57:18.028>
user = 'https://github.com/pppery'

bugs.python.org fields:

activity = <Date 2019-09-20.20:57:18.028>
actor = 'terry.reedy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2016-05-04.12:49:03.499>
creator = 'ppperry'
dependencies = []
files = []
hgrepos = []
issue_num = 26949
keywords = []
message_count = 9.0
messages = ['264861', '264863', '264867', '265097', '265101', '294124', '294125', '294775', '352889']
nosy_count = 3.0
nosy_names = ['terry.reedy', 'ppperry', 'louielu']
pr_nums = ['1711']
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26949'
versions = ['Python 3.9']