First and foremost, thanks for taking an interest in the issue I've encoutered.
I've added a simple "test" page to explain the problem I'm trying to describe.
On this test page, there should be a textbox and two divs, each of them colored in a different way.
Both are first going through a "disableSelection" operation (from jQuery UI Core).
But then, the yellow div (which I named "notrap", whereas the red one is named "trap") is given an additionnal attribute: "unselectable" is set to "on".
After the html page is loaded, I also bind an event handler on the textbox, for when it gains or loses focus, it should update which of the current document element active, should there be one (using document.activeElement).
Then a focus event is manually triggered on the textbox.
You shall notice that if you click the red div (trap), the document.activeElement is updated to the red div, whereas if I click on the yellow one, it remains still as the textbox.
I'm using IE 8.0.6001.18702IC on Windows XP SP3, by the way, but I think the issue might be pretty much the same on the other versions of IE 8 (the ones which are not under Windows XP). I have no idea whether it happens on other browsers, for I haven't given it any consideration.