fix: add unhandled exception handlers and robustness fallbacks for code completion

Author: PrashantUnity

Controls/CSharpCompletionData.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics;
 using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Layout;
@@ -29,9 +30,28 @@ public CSharpCompletionData(CSharpCompletionItem item)
 
     public double Priority => _item.Priority;
 
-    public object Content => _cachedContent ??= BuildContentControl();
+    public object Content => _cachedContent ??= SafeBuild(BuildContentControl, nameof(BuildContentControl));
 
-    public object Description => _cachedDescription ??= BuildDescriptionControl();
+    public object Description => _cachedDescription ??= SafeBuild(BuildDescriptionControl, nameof(BuildDescriptionControl));
+
+    private Control SafeBuild(Func<Control> build, string source)
+    {
+        try
+        {
+            return build();
+        }
+        catch (Exception ex)
+        {
+            Debug.WriteLine($"[CSharpCompletionData] {source} failed for '{_item.DisplayText}': {ex}");
+            return new TextBlock
+            {
+                Text = _item.DisplayText,
+                FontSize = 12,
+                Foreground = new SolidColorBrush(Color.Parse("#E6EDF3")),
+                Margin = new Thickness(4, 2)
+            };
+        }
+    }
 
     private Control BuildContentControl()
     {

Controls/CSharpEditorCompletionController.cs

@@ -1,7 +1,9 @@
 using System;
+using System.Diagnostics;
 using System.Threading;
 using System.Threading.Tasks;
 using Avalonia;
+using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Media;
 using Avalonia.Threading;
@@ -195,36 +197,72 @@ await Dispatcher.UIThread.InvokeAsync(() =>
                         data.Add(new CSharpCompletionData(item));
                     }
 
-                    _completionWindow.Closed += (s, e) =>
-                    {
-                        _completionWindow = null;
-                    };
+                    var window = _completionWindow;
+                    var completionList = window.CompletionList;
 
-                    _completionWindow.Show();
+                    void ApplySelection()
+                    {
+                        if (!ReferenceEquals(_completionWindow, window)) return;
+
+                        var currentCaret = _editor.CaretOffset;
+                        var currentText = _editor.Text ?? string.Empty;
+                        var effectiveQuery = initialQuery;
+                        if (currentCaret > startOffset && currentCaret <= currentText.Length)
+                        {
+                            effectiveQuery = currentText.Substring(startOffset, currentCaret - startOffset);
+                        }
+
+                        if (!string.IsNullOrEmpty(effectiveQuery))
+                        {
+                            completionList.SelectItem(effectiveQuery);
+                        }
+                        else if (data.Count > 0)
+                        {
+                            completionList.SelectedItem = data[0];
+                        }
+                    }
 
-                    var currentCaret = _editor.CaretOffset;
-                    var currentText = _editor.Text ?? string.Empty;
-                    var effectiveQuery = initialQuery;
-                    if (currentCaret > startOffset && currentCaret <= currentText.Length)
+                    void OnTemplateApplied(object? s, TemplateAppliedEventArgs e)
                     {
-                        effectiveQuery = currentText.Substring(startOffset, currentCaret - startOffset);
+                        completionList.TemplateApplied -= OnTemplateApplied;
+                        ApplySelection();
                     }
 
-                    if (!string.IsNullOrEmpty(effectiveQuery))
+                    window.Closed += (s, e) =>
+                    {
+                        completionList.TemplateApplied -= OnTemplateApplied;
+                        if (ReferenceEquals(_completionWindow, window)) _completionWindow = null;
+                    };
+
+                    window.Show();
+
+                    // AvaloniaEdit has a known upstream timing bug (AvaloniaUI/AvaloniaEdit
+                    // issues #308 and #357): TemplatedControl.ApplyTemplate() silently no-ops
+                    // if styling hasn't resolved CompletionList's ControlTemplate yet, leaving
+                    // its internal ListBox null - Show() does not guarantee it's ready. That's
+                    // harmless under light UI load (template resolves before the next frame)
+                    // but under FryPDF's heavier UI thread it can still be unresolved right
+                    // here, silently dropping the selection/highlight instead of crashing (the
+                    // crash itself is now caught by Dispatcher.UIThread.UnhandledException, but
+                    // that only stops the abort - it doesn't make the popup usable). Wait for
+                    // the template to genuinely finish applying before touching ListBox-backed
+                    // members if it isn't ready the instant Show() returns.
+                    if (completionList.ListBox != null)
                     {
-                        _completionWindow.CompletionList.SelectItem(effectiveQuery);
+                        ApplySelection();
                     }
-                    else if (data.Count > 0)
+                    else
                     {
-                        _completionWindow.CompletionList.SelectedItem = data[0];
+                        completionList.TemplateApplied += OnTemplateApplied;
                     }
                 });
             }
             catch (OperationCanceledException)
             {
             }
-            catch
+            catch (Exception ex)
             {
+                Debug.WriteLine($"[CSharpEditorCompletionController] TriggerCompletion failed: {ex}");
             }
         }, token);
     }

Runner/App.axaml.cs

@@ -1,13 +1,25 @@
+using System.Diagnostics;
 using Avalonia;
 using Avalonia.Controls.ApplicationLifetimes;
 using Avalonia.Markup.Xaml;
+using Avalonia.Threading;
 
 namespace PdfEditorApp.Plugins.CSharpEditor.Runner;
 
 public partial class App : Application
 {
     public override void Initialize()
     {
+        // See the matching handler in the hosted FryPDF app's App.axaml.cs for why this is
+        // needed: raw input is dispatched via Dispatcher.Send, which only offers this event
+        // as a way to intercept an exception thrown while handling a key/pointer event before
+        // it aborts the whole process.
+        Dispatcher.UIThread.UnhandledException += (_, e) =>
+        {
+            Debug.WriteLine($"[Dispatcher.UnhandledException] {e.Exception}");
+            e.Handled = true;
+        };
+
         AvaloniaXamlLoader.Load(this);
     }
 

Runner/Program.cs

@@ -1,3 +1,5 @@
+using System;
+using System.Threading.Tasks;
 using Avalonia;
 
 namespace PdfEditorApp.Plugins.CSharpEditor.Runner;
@@ -6,7 +8,14 @@ public static class Program
 {
     [STAThread]
     public static void Main(string[] args)
-        => BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+    {
+        AppDomain.CurrentDomain.UnhandledException += (_, e) =>
+            Console.Error.WriteLine($"[AppDomain.UnhandledException] {e.ExceptionObject}");
+        TaskScheduler.UnobservedTaskException += (_, e) =>
+            Console.Error.WriteLine($"[TaskScheduler.UnobservedTaskException] {e.Exception}");
+
+        BuildAvaloniaApp().StartWithClassicDesktopLifetime(args);
+    }
 
     public static AppBuilder BuildAvaloniaApp()
         => AppBuilder.Configure<App>()