Encode the issue title like the body

src/DiffEngineTray.Tests/IssueLauncherTests.cs

@@ -0,0 +1,49 @@
+/// <summary>
+/// The issue URL the tray opens after an error. Its title is built from a message that carries a
+/// file path, so it holds whatever characters the path does.
+/// </summary>
+public class IssueLauncherTests
+{
+    [Test]
+    public async Task A_hash_in_the_title_does_not_start_a_fragment()
+    {
+        // Perfectly ordinary in a solution directory, and everything after it used to be read by
+        // the browser as a fragment: no body, and a title ending at the hash
+        const string message = @"Cannot start. Failed to read settings: C:\code\C#\settings.json";
+
+        var url = IssueLauncher.BuildUrl(message);
+
+        await Assert.That(url).DoesNotContain("#");
+        await Assert.That(Query(url)["title"]).IsEqualTo(message);
+        await Assert.That(Query(url)["body"]).IsNotEmpty();
+    }
+
+    [Test]
+    public async Task An_ampersand_in_the_title_does_not_truncate_it()
+    {
+        const string message = @"Could not accept 'R&D.received.txt'";
+
+        var url = IssueLauncher.BuildUrl(message);
+
+        await Assert.That(Query(url)["title"]).IsEqualTo(message);
+        await Assert.That(Query(url)["body"]).IsNotEmpty();
+    }
+
+    /// <summary>
+    /// The body is encoded by its callers and handed over already escaped, so encoding the title
+    /// must not have changed what reaches GitHub as the body.
+    /// </summary>
+    [Test]
+    public async Task Keeps_the_body_it_is_given()
+    {
+        var url = IssueLauncher.BuildUrl("TheTitle", WebUtility.UrlEncode("\n * Action: TheAction"));
+
+        await Assert.That(Query(url)["body"]).Contains("* Action: TheAction");
+    }
+
+    static Dictionary<string, string> Query(string url) =>
+        url[(url.IndexOf('?') + 1)..]
+            .Split('&')
+            .Select(_ => _.Split('=', 2))
+            .ToDictionary(_ => _[0], _ => WebUtility.UrlDecode(_[1]));
+}

src/DiffEngineTray/IssueLauncher.cs

@@ -11,7 +11,15 @@ static IssueLauncher() =>
              """);
 
     public static void Launch() =>
-        LinkLauncher.LaunchUrl($"https://github.com/VerifyTests/DiffEngine/issues/new?title=TODO&body={defaultBody}");
+        LinkLauncher.LaunchUrl(BuildUrl("TODO"));
+
+    /// <summary>
+    /// The title is encoded like the body is. It is built from a message carrying a file path, and
+    /// a '#' in one started a fragment - dropping the body, and everything of the title after it -
+    /// while an '&amp;' started a parameter GitHub does not have, truncating the title there.
+    /// </summary>
+    internal static string BuildUrl(string title, string extraBody = "") =>
+        $"https://github.com/VerifyTests/DiffEngine/issues/new?title={WebUtility.UrlEncode(title)}&body={defaultBody}{extraBody}";
 
     public static void LaunchForException(string message, Exception exception)
     {
@@ -43,8 +51,7 @@ Open an issue on GitHub?
              {exception}
              ```
              """);
-        var url = $"https://github.com/VerifyTests/DiffEngine/issues/new?title={message}&body={defaultBody}{extraBody}";
-        LinkLauncher.LaunchUrl(url);
+        LinkLauncher.LaunchUrl(BuildUrl(message, extraBody));
     }
 
     public static void LaunchForException(string message)
@@ -71,8 +78,7 @@ Open an issue on GitHub?
 
               * Action: {message}
              """);
-        var url = $"https://github.com/VerifyTests/DiffEngine/issues/new?title={message}&body={defaultBody}{extraBody}";
-        LinkLauncher.LaunchUrl(url);
+        LinkLauncher.LaunchUrl(BuildUrl(message, extraBody));
     }
 
     static bool AskIfOpenIssue(string text)