Hi Fredrik,

As far as I'm aware, there's no out of the box way to do this but it is possible to do programmatically. You can create your own views to render content properties so, assuming you'd want this to apply to all XhtmlString properties, you can create a view called XhtmlString.cshtml in ~/Views/Shared/DisplayTemplates/ and modify the value of that property prior to rendering it like this:

@using EPiServer.Editor
@using EPiServer.Web.Mvc.Html
@model EPiServer.Core.XhtmlString

@{
    if (PageEditing.PageIsInEditMode)
    {
        Html.RenderXhtmlString(Model);
    }
    else
    {
        Html.RenderXhtmlString(Model.FixLinkTargets());
    }
}

To process the XhtmlString value so that it checks the content being linked and adds the appropriate target tag, I'd suggest using HTMLAgilityPack (nuget package) and using it to extract links. You can then use the IUrlResolver to get the content associated with the link and check its TargetFrameName value, setting any non-blank values to the target attribute (if a value's not already been set) like this:

public static class XhtmlStringHelpers
{
    public static XhtmlString FixLinkTargets(this XhtmlString xhtmlString)
    {
        if (xhtmlString == null)
        {
            return null;
        }

        var urlresolver = ServiceLocator.Current.GetInstance<IUrlResolver>();

        //use HTMLAgilityPack to get link tags
        var doc = new HtmlDocument();
        doc.LoadHtml(xhtmlString.ToInternalString());
        var linkTags = doc.DocumentNode.SelectNodes("//a");

        //no links = no further processing
        if (linkTags == null)
        {
            return xhtmlString;
        }

        foreach (var linkTag in linkTags)
        {
            var url = linkTag.GetAttributeValue("href", string.Empty);
            var target = linkTag.GetAttributeValue("target", string.Empty);

            //only process non-blank internal URLs without a target already set
            if (string.IsNullOrEmpty(url) || url.Contains("://") || !string.IsNullOrEmpty(target))
            {
                continue;
            }

            var urlBuilder = new UrlBuilder(url);
            var content = urlresolver.Route(urlBuilder);
            if (content == null)
            {
                continue;
            }
            if (content is PageData page && !string.IsNullOrEmpty(page.TargetFrameName))
            {
                linkTag.SetAttributeValue("target", page.TargetFrameName);
            }
        }

        return new XhtmlString(doc.DocumentNode.InnerHtml);
    }
}

Any fix for this issue ? We have just encountered the exact same situation in Episerver 11.3 TinyMCE 1.0

Would rather avoid the workaround if possible :)