In an existing contenteditable with“Hello World” in it, putting the cursor at the end of the editor, type isnt and let it auto-correct(presumably) to isn't ending in Hello World isn't with a space at the end. Note if auto-correct did or did not happen.
Chrome (Desktop)
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Android API 28
Android API 27
Android API 26
Android API 25
Android API 24
Android API 23
iOS 11.4