requestQueue get stuck when browser is experiencing intensive navigation

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Describe your context
Please provide us your environment so we can easily reproduce the issue.

• replace the result of pip list | grep dash below

dash                            1.5.1     
dash-auth                       1.2.0
dash-bio                        0.1.4
dash-bio-utils                  0.0.3rc3
dash-canvas                     0.0.11
dash-core-components            1.4.0
dash-cytoscape                  0.0.5
dash-dangerously-set-inner-html 0.0.2
dash-daq                        0.2.1
dash-flow-example               0.0.5
dash-html-components            1.0.1
dash-renderer                   1.2.0
dash-table                      4.5.0
plotly                          3.3.0

• if frontend related, tell us your Browser, Version and OS

  • OS: [Mac OS]
  • Browser [Chrome]
  • Version [77]

Describe the bug

This is kind of reproducible with the current dash-docs PR for 1.5.1 upgrade, but we don't have fixed steps to reproduce it each time.

To give a little background about issue. We introduced a new feature/parameter in the visit_and_snapshot API, what it does is to detect if window.store exists and makes sure all
the callbacks get fired and have a responseTime in the queue (refer to https://github.com/plotly/dash/blob/b3b9de9923d82ff1281889b3695d638e95bb1590/dash/testing/dash_page.py#L43-L48)

this mechanism works fine with the older dash version prior to the 1.5.x release. so the behavior is new. what I observed is that the tests in test_component.py are doing intensively url navigation in the browser, it basically goes through each sub section of dash docs in a very fast way, at some point, the request will get stuck with status = 'loading', a refresh of the page might solve the issue, but not always the case. the stuck point is always consistent on CircleCI and in a local setup. but not the same in the these two environments: i.e. circleci get stuck with /datatables/callbacks while local run might be /datatables/editable

this looks like a leak or something related to the recent async work

sample of a loading state https://circleci.com/api/v1.1/project/github/plotly/dash-docs/8326/output/106/0?file=true
DEBUG dash.testing.dash_page:dash_page.py:48 status of state rq [{u'status': 200, u'requestTime': 1572637143419, u'uid': u'ba5537d0-617e-8488-94b8-0743ae70833e', u'controllerId': u'chapter.children', u'rejected': False, u'responseTime': 1572637143451}, {u'status': 200, u'requestTime': 1572637143564, u'uid': u'ffd418a2-6583-0b41-68e1-f87978e29b89', u'controllerId': u'datatable-paging.data', u'rejected': False, u'responseTime': 1572637144000}, {u'status': 200, u'requestTime': 1572637143625, u'uid': u'00824a87-cc96-bd10-14e8-47c488901e4f', u'controllerId': u'table-paging-with-graph.data', u'rejected': False, u'responseTime': 1572637144090}, {u'status': u'loading', u'controllerId': u'table-sorting-filtering.data', u'uid': u'2dd02b0a-e358-ac9f-24c1-66289dbd0643', u'requestTime': 1572637143676}, {u'status': u'loading', u'controllerId': u'table-filtering.data', u'uid': u'511a97fc-603d-7a33-49dd-343365d2e978', u'requestTime': 1572637143719}, {u'status': u'loading', u'controllerId': u'table-paging-and-sorting.data', u'uid': u'4680495b-ed49-5470-089c-cc8927d40451', u'requestTime': 1572637143766}, {u'status': u'loading', u'controllerId': u'datatable-paging-page-count.data', u'uid': u'0f143cab-e84b-05bb-befb-05d20eb61c83', u'requestTime': 1572637143824}, {u'status': u'loading', u'controllerId': u'datatable-paging-page-count.page_count', u'uid': u'cdfd9b8e-5619-35d0-a1a3-f4a844dc8f92', u'requestTime': 1572637143872}, {u'status': u'loading', u'controllerId': u'table-multicol-sorting.data', u'uid': u'e086324e-52c1-44ac-aeae-84f308e2508b', u'requestTime': 1572637143918}, {u'status': 200, u'requestTime': 1572637144762, u'uid': u'd85d3baf-7a61-d324-b09a-6c7efc6485c0', u'controllerId': u'table-paging-with-graph-container.children', u'rejected': False, u'responseTime': 1572637145068}]

Inizia da test_component.py e dalla logica di visit_and_snapshot in dash/testing/dash_page.py intorno alle righe 43-48. Esegui i test del componente con una navigazione intensiva tra gli URL e ispeziona gli stati della coda delle richieste, in particolare le richieste che rimangono in caricamento. Il lavoro è completato quando i test terminano in modo affidabile senza lasciare callback nello stato di caricamento.