Edge 17 throws an error when pasting into editor

[engineering-this]

What is the purpose of this pull request?

Fixes

Does your PR contain necessary tests?

All patches which change the editor code must include tests. You can always read more
on PR testing,
how to set the testing environment and
how to create tests
in the official CKEditor documentation.

This PR contains

• Unit tests
• Manual tests

What changes did you make?

Testing for custom mime types (just by trying setData with such type) makes dataTransfer inaccessible in Edge, so as a workaround for upstream issue I've added if statement which will omit such test on Edge.

Note:
I have different error in console, than described in original issue, but with my fix it looks to work fine.

Closes #2169.

[Comandeer]

Isn't there any more elegant way to do it in Edge than browser sniffing? I'm wondering about using window.DataTransfer to create new DataTransfer object and test on it instead of already existing instance.

If it works, I'll add also unit test using the same technique.

[Comandeer]

Shouldn't it return true? As it's stated in comment to this method:

If true is returned, it means custom MIME types are not supported in the current browser

[engineering-this]

Oh, that negated return, good call. But now I am surprised that change actually makes it work.

[Comandeer]

Please add reference to the issues in our and Edge trackers.

[engineering-this]

I've tried such test for custom mime types on Edge:

var testTransfer = new DataTransfer();

testTransfer.setData( testType, testValue );
fallbackDataTransfer._isCustomMimeTypeSupported = testTransfer.getData( testType ) === testValue;
testTransfer.clearData( testType );

But such test still corrupts original data transfer, and Edge denies access to it's items. Also DataTransfer methods are poorly supported and it doesn't help in creating proper unit test.

[Comandeer]

There is already "Fixed Issues` section.

[Comandeer]

CKEDITOR.env.version is a float, not integer. In this case this conditional is targeting also 16.x Edge.

[Comandeer]

LGTM. I've just updated changelog entry and fixed a typo in comment.