Thanks a lot! Now it makes sense.
I end up with this solution: override HtmlEditor's method getDocMarkup like this:
Code:
getDocMarkup: function() {
var me = this,
h = me.iframeEl.getHeight() - me.iframePad * 2,
oldIE = (Ext.isIE6 || Ext.isIE7 || Ext.isIE8);
return Ext.String.format(
(oldIE?'':'<!DOCTYPE html>')
+ '<html><head><style type="text/css">'
+ (Ext.isOpera?'p{margin:0}':'')
+ 'body{border:0;margin:0;padding:{0}px;'
+ 'word-wrap: break-word;' // my line
+ (oldIE?'':'min-')
+ 'height:{1}px;box-sizing:border-box;-moz-box-sizing:border-box;-webkit-box-sizing:border-box;cursor:text;background-color:white;'
+ (Ext.isIE?'':'font-size:12px;font-family:{2}')
+ '}</style></head><body></body></html>'
, me.iframePad, h, me.defaultFont);
}