When using Telerik controls with Bootstrap theme and Bootstrap 3.x the radWindow title has cosmetic issue (see attached).
It appeared that Bootstrap global box-sizing: border-box is the reason.
Repro markup:
<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">
<!-- Optional theme -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css" integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp" crossorigin="anonymous">
<!-- Latest compiled and minified JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>
</head>
<body>
<form id="form1" runat="server">
<asp:ScriptManager runat="server" />
<telerik:RadWindow Skin="Bootstrap" ID="RadWindow1" runat="server" Title="Some RadWindow title" VisibleOnPageLoad="true" RenderMode="Lightweight">
<ContentTemplate>adadsa</ContentTemplate>
</telerik:RadWindow>
</form>
</body>
</html>
Bootstrap skin
Silk skin
The workaround is to set box-sizing: content-box in rwTitleWrapper:
<style>
.rwTitleWrapper
{
box-sizing: content-box !important;
}
</style>