The first version on the textarea only checked for new content on key up and key down events, which worked fine, even when pasting big blocks of text with Ctrl-V. But we totally forgot that there are many people out there who use contextual menus to copy/paste text. The checking function wouldn't be triggered until the user press a key again.
What we want is the script to respond at soon as you paste some texte, no matter how.
Internet explorer actually has a "onpaste" event, but the other browsers don't.
the onchange event is only fired when the input looses focus, if the content is different from when it got focus.
The solution is to set a periodical function, or "observer" which check regularly if the textarea content has changed.
In order to not have several observers running constantly, we only have one that we set on when an input get focus, and off when input looses focus.
The observer check for variation in the input's length every 500 milliseconds, which at the same time is not overkill, and gives the illusion there is no delay.