Editor crashes after toggling the read only mode while the selection is inside the editor

[Mgsy]

Is this a bug report or feature request? (choose one)

🐞 Bug report

💻 Version of CKEditor

Latest master.

📋 Steps to reproduce

1. Open the article sample.
2. Paste the following code to the console:

document.addEventListener( 'keydown', evt => {
    if ( evt.keyCode == '16' ) {
      editor.isReadOnly == true ? editor.isReadOnly = false : editor.isReadOnly = true;
    } 
} );

3. Put the caret inside the editor.
4. Press Shift twice.

✅ Expected result

There's no error in the console.

❎ Actual result

The editor crashes.

📃 Other details that might be useful

Error

ckeditorerror.js:46 Uncaught CKEditorError: cannot-change-view-tree: Attempting to make changes to the view when it is in an incorrect state: rendering or post-fixers are in progress. This may cause some unexpected behavior and inconsistency between the DOM and the view. Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-cannot-change-view-tree
    at View.change (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:78039:10)
    at InlineEditableUIView._updateIsFocusedClasses (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:97005:15)
    at InlineEditableUIView.on (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:96981:43)
    at InlineEditableUIView.fire (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:107797:29)
    at InlineEditableUIView.set [as isFocused] (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:109682:11)
    at updateBoundObservableProperty (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:110219:30)
    at bindings.forEach.binding (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:110246:7)
    at Set.forEach (<anonymous>)
    at FocusTracker.observable.listenTo (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:110245:15)
    at FocusTracker.fire (http://192.168.1.48:8125/ckeditor5-core/tests/manual/article.js:107797:29)

Notes

• It doesn't occur if the selection is outside of the editor.
• It's a regression.
• It could be easly reproduced with the collaboration. User is typing -> losing connection -> editor goes read only -> connection is restored -> editor leaves the read only mode -> crash.

[Mgsy]

cc @oskarwrobel

[oskarwrobel]

Looks like two async listeners try to change the view element at the same time:

• https://github.com/ckeditor/ckeditor5-ui/blob/b99450856def4f4ac567601bff7cdb3032cb93a9/src/editableui/editableuiview.js#L100
• https://github.com/ckeditor/ckeditor5-engine/blob/7815ab08a080dfb3387042aace1aa84473d2a473/src/view/view.js#L257

cc @oleq