I spent several hours trying to figure this out. I wish the documentation was better or there were a built-in function for this (like getSelectedText). If you want to retrieve the selected html (not plain text), try this:
Hope I save someone else the pain I went through.
editor.getSelection().getNative().createRange().htmlText
Hope I save someone else the pain I went through.
Re: Answer: how to get selected html from chkeditor
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Re: Answer: how to get selected html from ckeditor