In 4.2.0 try the following code:
In IE10 the third line is not visible (I guess the dialog isn't sizing correctly). It fails on both Classic and Neptune themes.
var msg = "xxxx xxxxxx xxxxxxxx xxxxxxxx xxxxxxx, 126.96.36.199 (ahahahahahahahahah)" +
"<br /><br />This is a second line of text" +
"<br /><br />This line is not visible in IE10";
So this weekend I've really proved to myself 4.2.0 isn't ready for me to use in production code... sigh.
Ext 4.2.0, IE10, Windows 7
PS. It works in Chrome