Fails on all Linux WR targets (debug, opt, shippable, asan):

16:06:10 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_ignore_charset.js | The charset parameter is ignored and UTF-8 is used. - Got null, expected "❤"
2713	16:06:11 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_nul.js | The NUL character has been preserved. - Got null, expected "\"foo_\u0000_bar\""
2747	16:06:11 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_object-type.js | There must be one row - Got 2, expected 1
2757	16:06:11 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_object-type.js | The label must be indicating an object - Got "", expected "{…}"
2813	16:06:12 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_row_selection.js | The row #1 is selected. - Got +0, expected 1
2823	16:06:12 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_row_selection.js | The row #2 is selected. - Got +0, expected 2
2833	16:06:12 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_row_selection.js | The row #3 is selected. - Got +0, expected 3
2930	16:06:58 INFO - TEST-UNEXPECTED-FAIL | devtools/client/jsonview/test/browser_jsonview_save_json.js | Test timed out -
10545	16:28:14 ERROR - Return code: 1
10546	16:28:14 ERROR - Got 9 unexpected statuses

https://treeherder.mozilla.org/logviewer?job_id=345318926&repo=try&lineNumber=2668

That log does no longer exist, but from https://treeherder.mozilla.org/push-health/push?repo=try&revision=ea241280baa96ac798c8d2f479154866fbe5e400, the tests that failed were

• devtools/client/jsonview/test/browser_jsonview_ignore_charset.js
• devtools/client/jsonview/test/browser_jsonview_object-type.js
• devtools/client/jsonview/test/browser_jsonview_save_json.js
• devtools/client/jsonview/test/browser_jsonview_nul.js
• devtools/client/jsonview/test/browser_jsonview_row_selection.js

These seem fine now, probably thanks to bug 1734940:

• devtools/client/jsonview/test/browser_jsonview_ignore_charset.js
• devtools/client/jsonview/test/browser_jsonview_nul.js

My patch for bug 1739320 seems to address

• devtools/client/jsonview/test/browser_jsonview_object-type.js
• devtools/client/jsonview/test/browser_jsonview_row_selection.js

So only devtools/client/jsonview/test/browser_jsonview_save_json.js will remain after that.

Seems clear the failures are caused by BrowserTestUtils.synthesizeMouseAtCenter.

Only skip browser_jsonview_save_json.js because of bug 1478596.

https://hg.mozilla.org/mozilla-central/rev/4165faa98243