FormDecorator changes focus when selection is dragged from one textbox to another in Internet Explorer: http://screencast.com/t/zwtVur2X
If you still experience the problem, you can apply the following solution:
Sys.Application.add_load(function () {
var browser = Telerik.Web.Browser;
if (browser.ie || browser.edge) {
var origin;
$telerik.$("input").on("mousedown", function (e) {
origin = e.target;
});
$telerik.$('input').on("mouseup", function (e) {
origin.focus();
});
}
});