src/PostIt/PostIt/Services/UiDispatcher.cs

@@ -0,0 +1,72 @@
+using System;
+using System.Threading.Tasks;
+using Avalonia.Threading;
+
+namespace PostIt.Services;
+
+/// <summary>
+/// Tiny marshalling helper around <see cref="Dispatcher.UIThread"/> so
+/// the rest of the codebase does not have to import Avalonia.Threading
+/// directly. We want exactly one place that decides "is the current
+/// thread the Avalonia UI thread, and if not, post there" so that
+/// <see cref="ObservableObject"/>-derived types (Settings, the various
+/// ViewModels) can fire <c>PropertyChanged</c> safely from background
+/// work — which is exactly the cross-thread case that previously blew
+/// up inside <c>DataValidationErrors.SetErrors</c> on Avalonia 11.
+///
+/// The helper is intentionally tiny: a sync post when we are off the
+/// UI thread, a no-op when we are already on it, and an async fire-
+/// and-forget variant for places where awaiting would deadlock the
+/// caller (e.g. <c>Settings.Load</c> continuation paths).
+/// </summary>
+public static class UiDispatcher
+{
+    /// <summary>
+    /// True when the calling thread is the Avalonia UI thread. Property
+    /// setters that touch bindings should check this before mutating
+    /// state; the safe path is <see cref="InvokeIfNeeded"/>.
+    /// </summary>
+    public static bool IsOnUiThread => Dispatcher.UIThread.CheckAccess();
+
+    /// <summary>
+    /// Run <paramref name="action"/> on the UI thread. If the caller is
+    /// already on the UI thread, run synchronously to preserve stack
+    /// traces and ordering; otherwise post to the dispatcher and wait.
+    /// Never throws on shutdown — a missing dispatcher is treated as
+    /// "best-effort skipped", matching Avalonia's own behaviour when
+    /// the application lifetime has been torn down.
+    /// </summary>
+    public static void InvokeIfNeeded(Action action)
+    {
+        if (action is null) return;
+        if (IsOnUiThread) { action(); return; }
+        try { Dispatcher.UIThread.Post(action, DispatcherPriority.Normal); }
+        catch (InvalidOperationException) { /* dispatcher gone, nothing to do */ }
+    }
+
+    /// <summary>
+    /// Fire-and-forget variant: schedules <paramref name="action"/> on
+    /// the UI thread but does not block the caller. Use this from
+    /// background workers (OIDC discovery, HTTP callbacks, file I/O)
+    /// where awaiting the dispatcher would deadlock the calling sync
+    /// context.
+    /// </summary>
+    public static void Post(Action action)
+    {
+        if (action is null) return;
+        try { Dispatcher.UIThread.Post(action, DispatcherPriority.Normal); }
+        catch (InvalidOperationException) { /* dispatcher gone */ }
+    }
+
+    /// <summary>
+    /// Awaitable variant. Useful inside <c>async</c> ViewModel methods
+    /// that must touch bindings only after the dispatcher has processed
+    /// a queued update (e.g. "load file then refresh observable state").
+    /// </summary>
+    public static Task InvokeAsync(Action action)
+    {
+        if (action is null) return Task.CompletedTask;
+        if (IsOnUiThread) { action(); return Task.CompletedTask; }
+        return Dispatcher.UIThread.InvokeAsync(action, DispatcherPriority.Normal).GetTask();
+    }
+}

src/Yavsc.Org/Views/Blogspot/Index.cshtml

@@ -70,7 +70,7 @@
         <div class="actiongroup">
             @if ((await AuthorizationService.AuthorizeAsync(User, post, new ReadPermission())).Succeeded)
             {
-                <a asp-action="Details" asp-route-id="@post.Id"  class="btn btn-light">Details</a>
+                <a asp-action="Details" asp-route-id="@((IBlogPost)post).Id"  class="btn btn-light">Details</a>
             }
             else
             {