Why the background thread's ouput will append to last active cell's output, not the actual corresponding cell?

[potoo0]

Description

the background thread's ouput will append to last active cell's output,

Reproduce

# cell1
from threading import Thread, current_thread
from time import sleep


def task():
    cnt = 3
    while cnt:
        print(f'task(cnt={cnt}) {current_thread()}')
        cnt -= 1
        sleep(1)

# cell2
Thread(target=task, name='t1').start();
# cell2 output
'''
task(cnt=3) <Thread(t1, started 140546848823040)>
''';

# cell3
Thread(target=task, name='t2').start();
# cell3 output
'''
task(cnt=3) <Thread(t2, started 140546857215744)>
task(cnt=2) <Thread(t1, started 140546848823040)>
task(cnt=2) <Thread(t2, started 140546857215744)>
task(cnt=1) <Thread(t1, started 140546848823040)>
task(cnt=1) <Thread(t2, started 140546857215744)>
''';

[JasonWeill]

This behavior may be by design. When a Jupyter notebook runs a cell, the cell runs to completion (you can see when [*] turns into a number like [2]). When future cells run, output is captured for their cell. Cells do not run in isolation from each other, and two cells don't run simultaneously, as far as I've observed.