Improve email suggestion handling and keyboard navigation

Enhanced the email suggestion feature by adding keyboard navigation support (`ArrowUp`, `ArrowDown`, `Enter`, `Escape`) and introducing `_selectedReceiverEmailSuggestion` to track the current selection. Updated methods to synchronize input and suggestions, pre-select matching suggestions, and reset the state when the popup is opened or closed. Improved error handling and ensured clean state management for better user experience.

EnvelopeGenerator.Server/EnvelopeGenerator.Server/Components/Pages/EnvelopeSenderEditorPage.razor

@@ -281,12 +281,14 @@
 
                 <div class="sender-receiver-popup__field">
                     <div class="sender-receiver-popup__label">E-Mail-Adresse</div>
-                    <DxTextBox Text="@_receiverDraftEmail"
+                    <div @onkeydown="OnReceiverEmailKeyDownAsync">
-                               TextChanged="OnReceiverEmailTextChangedAsync"
+                        <DxTextBox Text="@_receiverDraftEmail"
-                               NullText="name@beispiel.de"
+                                   TextChanged="OnReceiverEmailTextChangedAsync"
-                               ClearButtonDisplayMode="DataEditorClearButtonDisplayMode.Auto"
+                                   NullText="name@beispiel.de"
-                               BindValueMode="BindValueMode.OnInput"
+                                   ClearButtonDisplayMode="DataEditorClearButtonDisplayMode.Auto"
-                               CssClass="w-100" />
+                                   BindValueMode="BindValueMode.OnInput"
+                                   CssClass="w-100" />
+                    </div>
 
                     <div class="sender-receiver-popup__suggestions-shell">
                         @if (_receiverEmailSuggestions.Count > 0)
@@ -295,7 +297,7 @@
                             <DxListBox TData="string"
                                        TValue="string"
                                        Data="@_receiverEmailSuggestions"
-                                       Value="@_receiverDraftEmail"
+                                       Value="@_selectedReceiverEmailSuggestion"
                                        ValueChanged="OnReceiverEmailSuggestionSelectedAsync"
                                        SelectionMode="ListBoxSelectionMode.Single"
                                        CssClass="w-100" />
@@ -355,6 +357,7 @@
     bool _receiverPopupVisible;
     string _receiverDraftName = string.Empty;
     string _receiverDraftEmail = string.Empty;
+    string? _selectedReceiverEmailSuggestion;
     string? _receiverPopupValidationMessage;
     bool _isReceiverEmailSearchRunning;
     List<string> _receiverEmailSuggestions = [];
@@ -473,6 +476,7 @@
     {
         _receiverDraftName = string.Empty;
         _receiverDraftEmail = string.Empty;
+        _selectedReceiverEmailSuggestion = null;
         _receiverPopupValidationMessage = null;
         _receiverEmailSuggestions.Clear();
         _receiverPopupVisible = true;
@@ -482,6 +486,7 @@
     {
         _receiverPopupVisible = false;
         _receiverPopupValidationMessage = null;
+        _selectedReceiverEmailSuggestion = null;
         _isReceiverEmailSearchRunning = false;
     }
 
@@ -491,12 +496,56 @@
         _receiverPopupValidationMessage = null;
     }
 
+    async Task OnReceiverEmailKeyDownAsync(KeyboardEventArgs e)
+    {
+        if (_receiverEmailSuggestions.Count == 0)
+        {
+            if (e.Key == "Escape")
+                _selectedReceiverEmailSuggestion = null;
+
+            return;
+        }
+
+        var currentIndex = _selectedReceiverEmailSuggestion is null
+            ? -1
+            : _receiverEmailSuggestions.FindIndex(email => string.Equals(email, _selectedReceiverEmailSuggestion, StringComparison.OrdinalIgnoreCase));
+
+        if (e.Key == "ArrowDown")
+        {
+            var nextIndex = currentIndex < _receiverEmailSuggestions.Count - 1 ? currentIndex + 1 : 0;
+            await OnReceiverEmailSuggestionSelectedAsync(_receiverEmailSuggestions[nextIndex]);
+        }
+        else if (e.Key == "ArrowUp")
+        {
+            var nextIndex = currentIndex > 0 ? currentIndex - 1 : _receiverEmailSuggestions.Count - 1;
+            await OnReceiverEmailSuggestionSelectedAsync(_receiverEmailSuggestions[nextIndex]);
+        }
+        else if (e.Key == "Enter")
+        {
+            var selectedValue = currentIndex >= 0 && currentIndex < _receiverEmailSuggestions.Count
+                ? _receiverEmailSuggestions[currentIndex]
+                : _receiverEmailSuggestions.FirstOrDefault();
+
+            if (!string.IsNullOrWhiteSpace(selectedValue))
+            {
+                await OnReceiverEmailSuggestionSelectedAsync(selectedValue);
+                _receiverEmailSuggestions.Clear();
+            }
+        }
+        else if (e.Key == "Escape")
+        {
+            _receiverEmailSuggestions.Clear();
+            _selectedReceiverEmailSuggestion = null;
+        }
+    }
+
     Task OnReceiverEmailSuggestionSelectedAsync(string? value)
     {
         if (string.IsNullOrWhiteSpace(value))
             return Task.CompletedTask;
 
-        _receiverDraftEmail = value.Trim();
+        _selectedReceiverEmailSuggestion = value.Trim();
+        _receiverDraftEmail = _selectedReceiverEmailSuggestion;
         _receiverPopupValidationMessage = null;
         return Task.CompletedTask;
     }
@@ -504,6 +553,7 @@
     async Task OnReceiverEmailTextChangedAsync(string? value)
     {
         _receiverDraftEmail = value?.Trim() ?? string.Empty;
+        _selectedReceiverEmailSuggestion = _receiverDraftEmail;
         _receiverPopupValidationMessage = null;
 
         var searchVersion = ++_receiverEmailSearchVersion;
@@ -511,6 +561,7 @@
         if (string.IsNullOrWhiteSpace(_receiverDraftEmail) || _receiverDraftEmail.Length < 2)
         {
             _receiverEmailSuggestions.Clear();
+            _selectedReceiverEmailSuggestion = null;
             _isReceiverEmailSearchRunning = false;
             return;
         }
@@ -530,12 +581,18 @@
                 .OrderBy(email => email)
                 .Take(12)
                 .ToList();
+
+            _selectedReceiverEmailSuggestion = _receiverEmailSuggestions.FirstOrDefault(email =>
+                string.Equals(email, _receiverDraftEmail, StringComparison.OrdinalIgnoreCase));
         }
         catch (Exception ex)
         {
             Logger.LogWarning(ex, "Failed to load receiver email suggestions for {SearchTerm}", _receiverDraftEmail);
             if (searchVersion == _receiverEmailSearchVersion)
+            {
                 _receiverEmailSuggestions.Clear();
+                _selectedReceiverEmailSuggestion = null;
+            }
         }
         finally
         {