Thanks for the suggestion @Jinjinov! I've only tested this change on Linux and it looks like xdg-open isn't required, but I guess that it could vary from distro to distro. I'll give it a go.

I didn't have the time to check it on every OS, but I got it from here:

https://stackoverflow.com/a/43232486

and I left out url = url.Replace("&", "^&"); since https://github.com/dotnet/corefx/issues/10361 is closed.

I tested your code, but unfortunately clicking on <a href="https://www.google.com/">Google</a> doesn't call HandleWebRequest(object sender, string schema, string url, out string contentType)

Since I am making a Markdown app, I want the links in the user's Markdown notes to be clickable.

I am generating the Markdown with https://github.com/xoofx/markdig and changing the generation of <a> tags would be too difficult, so I have no control over the generated links.

Yes, the HandleWebRequest appears to be only called when the registered schema is used (i.e., app://). I'm also using Markdig and I am replacing the instances of "http://" and "https://" in links with "app://".

this.Html = Markdown.ToHtml(this.Text, pipeline);

// Photino workaround to allow links to be opened in external browser windows
this.Html = this.Html.Replace("<a href=\"http://", "<a href=\"app://");
this.Html = this.Html.Replace("<a href=\"https://", "<a href=\"app://");

Not ideal, I know, but it works for now.

Interesting workaround, thank you for sharing :)

But my core Blazor code is shared between several project:
Blazor WASM - for web
Blazor Maui - for Windows, Android, iOS and MacOS
Photino Blazor - for Linux

I would have to change the method that is using Markdig to work differently for Photino.

I don't know why, but in my app, <a href="app://www.google.com/">Google</a> doesn't call HandleWebRequest.

EDIT: because I am developing on Windows:

// On Windows, we can't use a custom scheme to host the initial HTML,
// because webview2 won't let you do top-level navigation to such a URL.
// On Linux/Mac, we must use a custom scheme, because their webviews
// don't have a way to intercept http:// scheme requests.
public static readonly string BlazorAppScheme = RuntimeInformation.IsOSPlatform(OSPlatform.Windows)
    ? "http"
    : "app";

public static readonly string AppBaseUri = $"{BlazorAppScheme}://localhost/";

It seems like that your solution works on Linux and Mac, but not on Windows.

I looked at the call stack and the call is coming from OnCustomScheme(string url, out int numBytes, out string contentType) in class PhotinoWindow in namespace PhotinoNET, but that method is called from native code.

I guess that a solution that would work on every OS would have to be implemented in both https://github.com/tryphotino/photino.Native and https://github.com/tryphotino/photino.NET

WebResourceRequestedCallback *CustomSchemeHandler; is declared in Photino.Native/Photino.h and used in Photino.Native/Photino.Linux.cpp and Photino.Native/Photino.Mac.mm and Photino.Native/Photino.Windows.cpp

I'm not quite sold on this solution tbh. It is confusing to have to use the app:// scheme to call web resources in my opinion. How about sending a message through window.external.sendMessage? You could bind a Javascript event to the a-tag and open the link inside the RegisterWebMessageReceivedHandler EventHandler on the C# end ...

Thanks @philippjbauer , that does indeed sound like a better solution and I agree that it shouldn't be merged in as is. I'll try your suggestion and let you guys know if it works.

I apologize for the off-topic comment, but this solution might help someone else who wants to open Markdown links in the default browser:

OnClickMarkdownExtension.cs

using Markdig;
using Markdig.Renderers;
using Markdig.Renderers.Html;
using Markdig.Syntax;
using Markdig.Syntax.Inlines;

namespace OpenMarkdownLinksInDefaultBrowser;

public class OnClickMarkdownExtension : IMarkdownExtension
{
    public void Setup(MarkdownPipelineBuilder pipeline)
    {
        pipeline.DocumentProcessed += AddOnClickToLinks;
    }

    public void AddOnClickToLinks(MarkdownDocument document)
    {
        foreach (LinkInline link in document.Descendants<LinkInline>())
        {
            link.GetAttributes().AddProperty("onclick", $"return openLink('{link.Url}');");
        }
    }

    public void Setup(MarkdownPipeline pipeline, IMarkdownRenderer renderer)
    {
    }
}

index.html

<script>
    function openLink(url) {
        event.preventDefault();
        DotNet.invokeMethodAsync('OpenMarkdownLinksInDefaultBrowser', 'OpenLink', url);
        return false;
    }
</script>

Program.cs

static void Main(string[] args)
{
    MarkdownPipeline _markdownPipeline = new MarkdownPipelineBuilder()
        .UseAdvancedExtensions()
        .UseSoftlineBreakAsHardlineBreak()
        .Use<OnClickMarkdownExtension>()
        .Build();
}

[JSInvokable]
public static void OpenLink(string url)
{
    Process.Start(new ProcessStartInfo(url) { UseShellExecute = true });
}

@andrew-bedford Looking forward to a new pull request. We're going to close this one for now. When you create the new pull request, please do it against the debug branch as we never merge directly into master. Thank you!