IDLE Shell - input() blocks output from another thread

GitHub fields:

assignee = 'https://github.com/terryjreedy'
closed_at = None
created_at = <Date 2020-02-22.10:29:37.734>
labels = ['3.8', 'expert-IDLE', 'type-bug', '3.7', '3.9']
title = 'IDLE threading + stdout/stdin observed blocking behavior'
updated_at = <Date 2020-05-27.00:58:53.959>
user = 'https://bugs.python.org/JohnSmith'

bugs.python.org fields:

activity = <Date 2020-05-27.00:58:53.959>
actor = 'terry.reedy'
assignee = 'terry.reedy'
closed = False
closed_date = None
closer = None
components = ['IDLE']
creation = <Date 2020-02-22.10:29:37.734>
creator = 'John Smith'
dependencies = []
files = ['48901', '49194']
hgrepos = []
issue_num = 39724
keywords = []
message_count = 12.0
messages = ['362456', '362467', '362468', '362469', '362484', '362497', '362504', '362528', '362535', '369993', '369994', '370038']
nosy_count = 4.0
nosy_names = ['gvanrossum', 'terry.reedy', 'taleinat', 'John Smith']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39724'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9']