Filed by: imoraru [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer?job_id=350880636&repo=mozilla-central
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/QkseF3YJQP-7PmXwsgJA2A/runs/0/artifacts/public/logs/live_backing.log

[task 2021-09-08T05:50:50.500Z] 05:50:50     INFO - TEST-START | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent
[task 2021-09-08T05:50:50.500Z] 05:50:50     INFO - Closing window 1c6c0520-fcbf-4c1b-b8d9-b2a678764f28
[task 2021-09-08T05:50:50.604Z] 05:50:50     INFO - {'actions': [{'type': 'none', 'actions': [{'type': 'pause', 'duration': 16}, {'type': 'pause', 'duration': 16}], 'id': '0'}, {'type': 'key', 'actions': [{'type': 'keyDown', 'value': 'a'}, {'type': 'keyUp', 'value': 'a'}], 'id': '1'}]}
[task 2021-09-08T05:50:50.667Z] 05:50:50     INFO - {'actions': [{'type': 'none', 'actions': [{'type': 'pause', 'duration': 16}, {'type': 'pause', 'duration': 16}], 'id': '2'}, {'type': 'key', 'actions': [{'type': 'keyDown', 'value': 'a'}, {'type': 'keyUp', 'value': 'a'}], 'id': '3'}]}
[task 2021-09-08T05:50:51.083Z] 05:50:51     INFO - {'actions': [{'type': 'none', 'actions': [{'type': 'pause', 'duration': 16}, {'type': 'pause', 'duration': 16}], 'id': '4'}, {'type': 'key', 'actions': [{'type': 'keyDown', 'value': 'a'}, {'type': 'keyUp', 'value': 'a'}], 'id': '5'}]}
[task 2021-09-08T05:50:51.167Z] 05:50:51     INFO - 
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - TEST-PASS | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent | Wait for load event 
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - TEST-FAIL | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent | execCommand("insertText", false, "typed value") in <input> - assert_equals: The value shouldn't be modified by "insertText" command expected "default value" but got "typed value"
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - @http://web-platform.test:8000/editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent:51:18
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:2087:25
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - promise_test/tests.promise_tests</<@http://web-platform.test:8000/resources/testharness.js:638:36
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - promise_test/tests.promise_tests<@http://web-platform.test:8000/resources/testharness.js:637:20
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - TEST-FAIL | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent | execCommand("delete") in <input> - assert_equals: The value shouldn't be modified by "delete" command expected "default value" but got ""
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - @http://web-platform.test:8000/editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent:63:18
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - Test.prototype.step@http://web-platform.test:8000/resources/testharness.js:2087:25
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - promise_test/tests.promise_tests</<@http://web-platform.test:8000/resources/testharness.js:638:36
[task 2021-09-08T05:50:51.170Z] 05:50:51     INFO - promise_test/tests.promise_tests<@http://web-platform.test:8000/resources/testharness.js:637:20
[task 2021-09-08T05:50:51.171Z] 05:50:51     INFO - TEST-UNEXPECTED-PASS | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent | <input> is hidden by "keydown" event listener - expected FAIL
[task 2021-09-08T05:50:51.171Z] 05:50:51     INFO - TEST-INFO | expected FAIL
[task 2021-09-08T05:50:51.173Z] 05:50:51     INFO - ..
[task 2021-09-08T05:50:51.173Z] 05:50:51     INFO - TEST-OK | /editing/other/edit-in-textcontrol-immediately-after-hidden.tentative.html?editor=input&hide-target=parent | took 662ms
[task 2021-09-08T05:50:51.175Z] 05:50:51     INFO - PID 8236 | 1631080251169	Marionette	INFO	Stopped listening on port 51685
[task 2021-09-08T05:50:51.221Z] 05:50:51     INFO - PID 8236 | JavaScript error: resource:///modules/Interactions.jsm, line 201: NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIUserIdleService.removeIdleObserver]
[task 2021-09-08T05:50:51.539Z] 05:50:51     INFO - PID 8236 | ###!!! [Parent][RunMessage] Error: Channel closing: too late to send/recv, messages will be lost
[task 2021-09-08T05:50:51.604Z] 05:50:51     INFO - Browser exited with return code 0
[task 2021-09-08T05:50:51.608Z] 05:50:51     INFO - Closing logging queue
[task 2021-09-08T05:50:51.608Z] 05:50:51     INFO - queue closed
[task 2021-09-08T05:50:51.673Z] 05:50:51     INFO - Application command: Z:\task_163107905981814\build\application\firefox\firefox.exe -marionette about:blank --wait-for-browser -profile C:\Users\task_163107905981814\AppData\Local\Temp\tmp2tic54hs
[task 2021-09-08T05:50:51.679Z] 05:50:51     INFO - PID 1500 | 1631080247985	Marionette	INFO	Marionette enabled
[task 2021-09-08T05:50:51.682Z] 05:50:51     INFO - PID 1500 | console.warn: SearchSettings: "get: No settings file exists, new profile?" (new NotFoundError("Could not open the file at C:\\Users\\task_163107905981814\\AppData\\Local\\Temp\\tmpf1a067oc\\search.json.mozlz4", (void 0)))
[task 2021-09-08T05:50:51.683Z] 05:50:51     INFO - PID 1500 | [Parent 9196, IPC I/O Parent] WARNING: file /builds/worker/checkouts/gecko/ipc/chromium/src/base/process_util_win.cc:167
[task 2021-09-08T05:50:51.683Z] 05:50:51     INFO - PID 1500 | 1631080250403	Marionette	INFO	Listening on port 51722
[task 2021-09-08T05:50:51.684Z] 05:50:51     INFO - Starting runner
[task 2021-09-08T05:50:52.758Z] 05:50:52     INFO - TEST-START | /editing/other/editing-around-select-element.tentative.html?forwardDelete

Hmm, it probably means that something randomly runs while dispatching a keydown element and it flushes pending notifications which includes destroying the target editor. So editor won't handle it and unexpectedly passes the test.