At present, when IE is in Compatibility Mode the UI for ASP.NET AJAX controls use the Classic RenderMode in all cases because of the User Agent string the browser provides. If RenderMode="Lightweight" and the X-UA-Compatible meta tag with value "IE=edge" is present on the server, the controls will render in the Lightweight mode instead of falling back to Classic. This will also apply to RenderMode="Auto".
We ship fixes and improvements in the first official release after their implementation (http://docs.telerik.com/devtools/aspnet-ajax/licensing/old-versions-support-policy), so, I am afraid, it will not be available as a patch.
Thanks Marin. Will this be available as a patch to 2017.1.118? Cheers.
You are completely right, Simon, we will fix this for R2 2017 SP2
Please note, the code does a case sensitive compare of "IE=Edge" of the X-UA-Compatible header so the value must be "IE=Edge" not "IE=edge". This conflicts with the Microsoft documentation - https://msdn.microsoft.com/en-us/library/ff955275(v=vs.85).aspx This is a bug and the code should be doing a cases insensitive compare I think. Also not that the header needs to be added early in the page lifecycle in WebForms before the controls are created e.g. PreInit.