Mock synchronisation issues on Windows only (Linux/OSX fine)

Describe your context

dash                                    1.1.1
dash-bootstrap-components               0.7.0
dash-core-components                    1.1.1
dash-html-components                    1.0.0
dash-renderer                           1.0.0
dash-table                              4.1.0
selenium                                3.141.0

Previously tested with dash 1.0.2 -- same behaviour observed.

• Browser, Version and OS

  • OS: Windows 10 Pro
  • Browser: Chrome
  • Version: 76.0.3809.87, 75.0.3770.80 (matched chromedriver version used in both cases)
  • Issue does NOT occur using same Chrome versions on Manjaro Linux or OSX
  • Reproduced on two different machines/Windows 10 installations

Describe the bug

Unit tests using dash-testing / selenium clear_input() / send_keys() methods drive browser input as expected (verified visually -- inserting artificial time.sleep() statements on occasion to be sure). Mocked functions capture some but not all invocations. In some cases, inserting additional calls to clear_input() appears to cause some internal (?inter-process?) synchronisation of the mock to occur, capturing the outstanding invocations. However this is inconsistent -- the workaround shown in the self-contained example below works consistently in that example, but not in other very similar tests in our codebase.

Expected behavior

Mocks record invocations from all callback invocations without need for additional attempts to flush/synchronise. I.e. behaviour consistent with expectations / verified behaviour on Linux and OSX.

Self-contained example

import re
import random
import time
import pytest

import dash
import dash_html_components as html
import dash_core_components as dcc
from dash.dependencies import Input, Output, State

import platform

DEFAULT_VALUE = 20
NEW_VALUE = -20

def _build_app_layout():
    app = dash.Dash(__name__)
    app.layout = html.Div(id='top',children=[
      dcc.Input(id='some_input', value=str(DEFAULT_VALUE)),
      html.Div(id='alert_container')
    ])
    @app.callback(Output('alert_container', 'children'),
                  [Input('some_input', 'value')])
    def validate_input(new_value: str):
        if re.match(r'[+-]?\d+',new_value):
            #Nonsense call so we can count num invocations
            _throwaway = random.randint(int(new_value),DEFAULT_VALUE)
            return 'Unmodified' if new_value == str(DEFAULT_VALUE) else 'Modified'
        #Nonsense call so we can count num invocations
        _throwaway = random.randint(0,DEFAULT_VALUE)
        return 'Invalid'
    return app


@pytest.mark.webtest
def test_should_validate_inputs_when_modified(mocker, dash_duo):
    """This is a self-contained test intended to reproduce a problem we've been
    having, where mocked functions invoked from dash callbacks can be verified on
    Linux and OSX, but not on Windows.
    """
    # Given
    mocker.patch('random.randint', return_value=DEFAULT_VALUE)
    dash_duo.start_server(_build_app_layout())
    input_element = dash_duo.find_element('input#some_input')
    initial_value = input_element.get_attribute('value')

    # When
    dash_duo.clear_input(input_element)
    input_element.send_keys(f'{NEW_VALUE}')
    dash_duo.wait_for_contains_text('input#some_input', str(NEW_VALUE), timeout=5)
    # Uncommenting the next two lines causes test to pass on Windows
    #if platform.system() == 'Windows':
    #    dash_duo.clear_input(input_element)

    # Then
    # ... over-elaborated assertions to see error detail
    random.randint.assert_has_calls([
        mocker.call(DEFAULT_VALUE, DEFAULT_VALUE),
        mocker.call(0, DEFAULT_VALUE),# <clear>
        mocker.call(0, DEFAULT_VALUE),# -
        mocker.call(-2, DEFAULT_VALUE),
        mocker.call(-20, DEFAULT_VALUE)
    ])
    # ... actual assertion if we resolve this issue
    random.randint.assert_called_with(NEW_VALUE, DEFAULT_VALUE)

Start with the self-contained pytest test using dash_duo, clear_input(), send_keys(), and the mocked random.randint calls. Compare callback mock synchronization on Windows with Linux and OSX, then verify that all expected invocations are captured without the extra clear_input() workaround.