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.
The workaround is to set box-sizing: content-box in rwTitleWrapper
It could easily reproduced here https://demos.telerik.com/aspnet-ajax/window/examples/browserdialogboxes/defaultcs.aspx
Please fix in your next release(s).