Fix obscure list traversal issue

If you select a note, delete it, select some nearby lines, and hit
undo, then the next time you hit up/down arrow the list will jump
back to the first line.  The workaround for this appears to be to
set the focus on a selected ListViewItem from an obscure event.

Maybe there's a simpler way that I just missed?  This is absurd.
(Yay WPF.)

SourceGenWPF/MainController.cs

@@ -1322,6 +1322,7 @@ namespace SourceGenWPF {
 
         public void HandleCodeListDoubleClick(int row, int col) {
             Debug.WriteLine("DCLICK: row=" + row + " col=" + col);
+            mMainWin.CodeListView_DebugValidateSelectionCount();
 
             // Clicking on some types of lines, such as ORG directives, results in
             // specific behavior regardless of which column you click in.  We're just
@@ -2267,7 +2268,7 @@ namespace SourceGenWPF {
         ///   is selected.</param>
         public SelectionState UpdateSelectionState() {
             int selCount = mMainWin.CodeListView_GetSelectionCount();
-            Debug.WriteLine("SelectionChanged: selCount=" + selCount);
+            Debug.WriteLine("UpdateSelectionState: selCount=" + selCount);
 
             SelectionState state = new SelectionState();
 

SourceGenWPF/WpfGui/MainWindow.xaml.cs

@@ -24,6 +24,7 @@ using System.Reflection;
 using System.Runtime.CompilerServices;
 using System.Windows;
 using System.Windows.Controls;
+using System.Windows.Controls.Primitives;
 using System.Windows.Data;
 using System.Windows.Input;
 using System.Windows.Media;
@@ -91,6 +92,10 @@ namespace SourceGenWPF.WpfGui {
             // https://dlaa.me/blog/post/9425496 to re-auto-size after data added (this may
             //  not work with virtual items)
 
+            // Obscure tweak to make the arrow keys work right after a change.
+            codeListView.ItemContainerGenerator.StatusChanged +=
+                ItemContainerGenerator_StatusChanged;
+
             mMainCtrl = new MainController(this);
 
             AddMultiKeyGestures();
@@ -518,8 +523,8 @@ namespace SourceGenWPF.WpfGui {
             // Notify MainController that the selection has changed.
             mMainCtrl.SelectionChanged();
 
-            Debug.Assert(CodeDisplayList.SelectedIndices.DebugValidateSelectionCount(
-                codeListView.SelectedItems.Count));
+            // Don't try to call CodeDisplayList.SelectedIndices.DebugValidateSelectionCount()
+            // here.  Events arrive while pieces are still moving.
 
             //Debug.WriteLine("SelectionChanged took " +
             //    (DateTime.Now - startWhen).TotalMilliseconds + " ms");
@@ -654,7 +659,46 @@ namespace SourceGenWPF.WpfGui {
             //    (DateTime.Now - startWhen).TotalMilliseconds + " ms");
         }
 
-        public int CodeListView_GetTopIndex() {
+        public void CodeListView_DebugValidateSelectionCount() {
+            Debug.Assert(CodeDisplayList.SelectedIndices.DebugValidateSelectionCount(
+                codeListView.SelectedItems.Count));
+        }
+
+        /// <summary>
+        /// Sets the focus to the ListViewItem identified by SelectedIndex.  This must be done
+        /// when the ItemContainerGenerator's StatusChanged event fires.
+        /// </summary>
+        /// <remarks>
+        /// Steps to cause problem:
+        ///  1. select note
+        ///  2. delete note
+        ///  3. select nearby line
+        ///  4. edit > undo
+        ///  5. hit the down-arrow key
+        ///
+        /// Without this event handler, the list jumps to line zero.  The original article
+        /// was dealing with a different problem, where you'd have to hit the down-arrow twice
+        /// to make it move, because the focus was on the control rather than the item.  The
+        /// same fix seems to apply for this issue as well.
+        ///
+        /// From http://cytivrat.blogspot.com/2011/05/selecting-first-item-in-wpf-listview.html
+        /// </remarks>
+        private void ItemContainerGenerator_StatusChanged(object sender, EventArgs e) {
+            if (codeListView.ItemContainerGenerator.Status == GeneratorStatus.ContainersGenerated) {
+                int index = codeListView.SelectedIndex;
+
+                if (index >= 0) {
+                    ListViewItem item =
+                        (ListViewItem)codeListView.ItemContainerGenerator.ContainerFromIndex(index);
+
+                    if (item != null) {
+                        item.Focus();
+                    }
+                }
+            }
+        }
+
+    public int CodeListView_GetTopIndex() {
             return codeListView.GetTopItemIndex();
         }