Undo/redo widget bug

[Andrew]

Steps to reproduce

1. go to the simplebox example at: ​http://ckeditor.com/demo#widgets
2. enter a fair amount of text into the content section of the simplebox example
3. after N amount of redo/undo (CTRL+Z / CTRL + Y on the linked page but also the undo/redo buttons on my own project) the error 'IndexSizeError: Index or size is negative or greater than the allowed amount' will be thrown for any undo/redo from then on.

Expected result

Expected to undo/redo without errors

Actual result

Errors are thrown when undo/redo is used after a certain amount of calls within the editable area of an editable widget

Other details (browser, OS, CKEditor version, installed plugins)

As mention above, this happens with both CTRL+Z/CTRL+Y and undo/redo buttons on the editor. Browsers: Chrome 50.0.2661.102 Firefox: 46.0.1 IE 11: 11.0.0

This bug will be triggered 100% of the time albeit with variable amount of undo/redo calls (can be within the first 5 or so or up to 20+ calls).

[Andrew]

Note: this error will also continue to be thrown if you also undo/redo text outside of the widget after the initial error is thrown from within the widget.

[Jakub Ś]

What does it mean fair amount of content. I have tried entering huge/small/medium amount of content pressed und/redo like crazy (although this isn't standard usage) and nothing happened.

Please provide screen cast which shows how to reproduce this issue in CKEditor demo.