I found the problem. You can see a video of how this works at https://goo.gl/i6kwTF. Also, a working project is attached which you can verify on your end.
There are two issues being introduced by your code.
- You have specified a CSS rule of overflow-y:hidden for a div element that is causing bottom border to be hidden. RadAlert has multiple div elements in its rendered html and this CSS rule therefore gets applied to each div element in radalert. Suc a CSS rule clips the bottom content of radalert div causing the bottom border to be not visible. In this case, you need to remove this style in mntThirdPartySpend.aspx as shown below. Remember never to hide overflow when using RadAutoSizing library since content is then clipped. So whichever page you include this library, all overflow hidden should be removed.
The way you have used RadScriptManager is correct since it needs to be included for each aspx page and you are not using user controls. So if you want to have radalert autosized in mainwrapper also on all pages of your app then just include the script in all pages within RadScriptManager and everything will work, but keep above two points in mind.
Make sure when opening Radalert from client-side that page does not post back
"Show QlikView Add Group"
"ShowGroup(); return false;"
"Show user's Qlikview group and user's domain"
Remove overflow hidden CSS rules when using autosizing library
body, div, html