When typing the content using visual editor in Edit Post screen, texts are automatically folded at the width of text area - that is ok.
But keep widening the browser width so that the width of textarea is more than around 606px - the line of text is folded at as if it is still at this width.
It is hard to explain in words. Please see the screen shot.
Latest Chrome, Mac OS X. Installed After the Deadline and Multibyte patch were switched off.