Rohit1 ckeditor4 load eventlistener leak fix

[rohit1]

What is the purpose of this pull request?

Does your PR contain necessary tests?

All patches that 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

Did you follow the CKEditor 4 code style guide?

Your code should follow the guidelines from the CKEditor 4 code style guide which helps keep the entire codebase consistent.

• PR is consistent with the code style guide

What is the proposed changelog entry for this pull request?

* [#<ISSUE_NUMBER>](https://github.com/ckeditor/ckeditor4/issues/<ISSUE_NUMBER>): Describe the purpose of the pull request in a few simple sentences.

What changes did you make?

added removeEventListener() in onReady callback function

Which issues does your PR resolve?

Closes #4273.

[f1ames]

Thank you for contribution @rohit1!

Your pull request is missing tests, see our Contributing Code guide for more details.

I think unit tests would be enough here. If you will have any problems writing them, just feel free to reach out so we can assist or take over this PR.

[github-actions]

It's been a while since we last heard from you. We are marking this pull request as stale due to inactivity. Please provide the requested feedback or the pull request will be closed after next 7 days.

[f1ames]

Ok, taking over this PR to add tests.

[f1ames]

Rebased and retargeted onto latest master.

[f1ames]

Unfortunately, changes in ckeditor_base.js requires special caution due to some convoluted way in which fixes should be applied there, specifically:

ckeditor4/core/ckeditor_base.js

Lines 11 to 17 in aa1e0e7

	// #### Compressed Code
	// Compressed code in ckeditor.js must be be updated on changes in the script.
	// The Closure Compiler online service should be used when updating this manually:
	// http://closure-compiler.appspot.com/
	// #### Raw code
	// ATTENTION: read the above "Compressed Code" notes when changing this code.

So apart from changes in this file, the changes then needs to be compressed and moved to ckeditor.js... 😬 Fortunately, the closure compiler linked there is still online (probably could be replaced with other tool). Not sure if it works the same as some time ago, OTOH it shouldn't hopefully break anything.

[f1ames]

Ok, I have to ignore unit tests on IE8 due to sinon stubbing issue (see this commit cfc5283). Also IE8 is ignored outside of bender.test which seems to be working fine (apart from return which is added to prevent further code execution), but I assume it's not correct use... However, it simplifies tests (due to different methods for listeners attaching in IE8).

[f1ames]

All tests in IE8 are also green so I guess nothing exploded 😅

[Comandeer]

LGTM!

f1ames took over this PR