Test 'tests/plugins/tableselection/integrations/clipboard/pasteflow' crashes when browser viewport focused

[f1ames]

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Bug

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Extracted from #962.
Similar to #945.

Provide detailed reproduction steps (if any)

The test tests/plugins/tableselection/integrations/clipboard/pasteflow started to fail (or rather break bender run in the same manner as mentioned in #945), after changes introduced in #962. However it only fails when browser viewport is focused (focusing browser window or browser URL field makes test run smoothly).

Also when changing tests order, from:

'test paste flow (tabular content)': function( editor, bot ) {
	testPasteFlow( bot, 'tabular-paste', '2cells1row' );
},

'test paste flow (non-tabular content)': function( editor, bot ) {
	testPasteFlow( bot, 'nontabular-paste', 'paragraph' );
}

to

'test paste flow (non-tabular content)': function( editor, bot ) {
	testPasteFlow( bot, 'nontabular-paste', 'paragraph' );
},

'test paste flow (tabular content)': function( editor, bot ) {
	testPasteFlow( bot, 'tabular-paste', '2cells1row' );
}	

it works fine despite the browser focus.

Other details

• Browser: Edge 16+
• OS: Windows 10
• CKEditor version: 4.7.3 with Workaround for Edge not supporting custom MIME types in dataTransfer.setData #962
• Installed CKEditor plugins: clipboard, tableselection, undo

[mlewand]

Interestingly I can no longer repro it with (t/468 branch):

Microsoft Edge 41.16299.15.0
Microsoft EdgeHTML 16.16299

[f1ames]

Just wanted to mention that simplest workaround (changing test order) is already present on #1153 (73a07fc) and this issue was to analyze the problem in more depth.

I will check if for the former test, the issue is still present on Edge 16.16299. If so I suggest to use the already present workaround as @msamsel fix is quite similar. If test will be passing on new Edge version, I will revert those change from #1153.

[f1ames]

Confirmed that this issue no longer occurs in Edge 41.16299. I reverted the workaround d9e2162 on t/468.