Fix auto complete not working in middle of text

src/Decho/ViewModels/AutocompleteController.cs

@@ -31,7 +31,9 @@ public sealed class AutocompleteController(IEnumerable<AutocompleteProvider> pro
 
     public int TriggerCharIndex { get; private set; }
 
-    public void Update(string text)
+    public int WordEndIndex { get; private set; }
+
+    public void Update(string text, int caretIndex)
     {
         if (string.IsNullOrEmpty(text) || _providers.Count == 0)
         {
@@ -39,7 +41,8 @@ public sealed class AutocompleteController(IEnumerable<AutocompleteProvider> pro
             return;
         }
 
-        Match match = TriggerPattern.Match(text);
+        int clampedCaret = Math.Clamp(caretIndex, 0, text.Length);
+        Match match = TriggerPattern.Match(text[..clampedCaret]);
         if (!match.Success)
         {
             Reset();
@@ -58,6 +61,7 @@ public sealed class AutocompleteController(IEnumerable<AutocompleteProvider> pro
         ActiveProvider = provider;
         FilterText = filter;
         TriggerCharIndex = match.Index;
+        WordEndIndex = match.Index + match.Length;
 
         List<string> items = provider.ItemsSource().ToList();
         List<string> filtered = items

src/Decho/ViewModels/MessageComposerViewModel.cs

@@ -106,8 +106,6 @@ public sealed class MessageComposerViewModel : ViewModelBase
                     (draft, hasFiles, isReadOnly) =>
                     (!string.IsNullOrWhiteSpace(draft) || hasFiles) && !isReadOnly);
         SendCommand = ReactiveCommand.Create(Send, canSend);
-
-        _ = this.WhenAnyValue(x => x.Draft).Subscribe(OnDraftChanged);
     }
 
     public void UpdateAvailableUsers(IEnumerable<UserViewModel> users)
@@ -179,17 +177,24 @@ public sealed class MessageComposerViewModel : ViewModelBase
         return _commandHandler?.IsCommand(input) ?? input.StartsWith('/');
     }
 
-    public void InsertAutocomplete(string item)
+    public int InsertAutocomplete(string item)
     {
         string? insertion = Autocomplete.GetInsertion(item);
         if (insertion is null)
         {
-            return;
+            return -1;
         }
 
         string before = Draft[..Autocomplete.TriggerCharIndex];
-        Draft = $"{before}{insertion}";
+        string after = Draft[Autocomplete.WordEndIndex..];
+        if (after.Length > 0 && char.IsWhiteSpace(after[0]))
+        {
+            insertion = insertion.TrimEnd();
+        }
+
+        Draft = $"{before}{insertion}{after}";
         Autocomplete.Reset();
+        return before.Length + insertion.Length;
     }
 
     public void ClearReplyTarget()
@@ -226,8 +231,8 @@ public sealed class MessageComposerViewModel : ViewModelBase
             : string.Empty;
     }
 
-    private void OnDraftChanged(string? draft)
+    public void UpdateAutocomplete(string text, int caretIndex)
     {
-        Autocomplete.Update(draft ?? string.Empty);
+        Autocomplete.Update(text, caretIndex);
     }
 }

src/Decho/Views/MessageComposerView.axaml.cs

@@ -1,3 +1,4 @@
+using Avalonia;
 using Avalonia.Controls;
 using Avalonia.Input;
 using Avalonia.Interactivity;
@@ -29,6 +30,11 @@ public partial class MessageComposerView : UserControl
         // Intercept Enter before the TextBox processes it (AcceptsReturn=True would insert a newline)
         MessageTextBox.AddHandler(InputElement.KeyDownEvent, OnTextBoxTunnelingKeyDown, RoutingStrategies.Tunnel);
 
+        MessageTextBox.GetObservable(TextBox.TextProperty)
+            .Subscribe(_ => UpdateAutocomplete(MessageTextBox.Text ?? string.Empty, MessageTextBox.CaretIndex));
+        MessageTextBox.GetObservable(TextBox.CaretIndexProperty)
+            .Subscribe(_ => UpdateAutocomplete(MessageTextBox.Text ?? string.Empty, MessageTextBox.CaretIndex));
+
         _spellChecker = new TextBoxSpellChecker(SpellCheckerConfig.Create("en_GB"));
         _spellChecker.Initialize(MessageTextBox);
     }
@@ -47,6 +53,12 @@ public partial class MessageComposerView : UserControl
         }
     }
 
+    private void UpdateAutocomplete(string text, int caretIndex)
+    {
+        MessageComposerViewModel? vm = this.GetDataContext<MessageComposerViewModel>();
+        vm?.UpdateAutocomplete(text, caretIndex);
+    }
+
     private void OnTextBoxKeyDown(object? sender, KeyEventArgs e)
     {
         MessageComposerViewModel? vm = this.GetDataContext<MessageComposerViewModel>();
@@ -137,8 +149,11 @@ public partial class MessageComposerView : UserControl
             return;
         }
 
-        vm.InsertAutocomplete(item);
-        MessageTextBox.CaretIndex = MessageTextBox.Text?.Length ?? 0;
+        int newCaret = vm.InsertAutocomplete(item);
+        if (newCaret >= 0)
+        {
+            MessageTextBox.CaretIndex = newCaret;
+        }
         _ = MessageTextBox.Focus();
     }