SharePoint CSOM get absolute url from FileRef

 This one liner can return absolute url of an item using its FileRef property

var absoluteUrl = new Uri(context.Url).GetLeftPart(UriPartial.Authority) + serverRelativeUrl;

 Copied from https://www.red-gate.com/simple-talk/blogs/getting-the-absolute-url-of-a-file-in-csom/