Do not throw the reader to the top for selecting what is already selected

src/DiffEngineViewer.Tests/ReselectTests.cs

@@ -0,0 +1,73 @@
+/// <summary>
+/// Selecting the entry that is already selected. A left click on the highlighted row, a right
+/// click to open its menu, and a focus naming it all arrive as a selection, and the reader is part
+/// way down a comparison while they do.
+/// </summary>
+public class ReselectTests
+{
+    [Test]
+    public async Task Opening_the_menu_on_the_entry_being_read_keeps_the_scroll()
+    {
+        var state = Scrolled();
+        var row = VisibleRowOf(state, state.Selected);
+
+        var opened = ViewerSession.OpenMenu(state, row);
+
+        await Assert.That(opened.Menu).IsNotNull();
+        await Assert.That(opened.ScrollTop).IsEqualTo(state.ScrollTop);
+    }
+
+    [Test]
+    public async Task Focusing_the_entry_being_read_keeps_the_scroll()
+    {
+        var state = Scrolled();
+
+        var focused = ViewerSession.SelectKey(state, state.Current!.Key);
+
+        await Assert.That(focused.ScrollTop).IsEqualTo(state.ScrollTop);
+    }
+
+    /// <summary>
+    /// A different entry is a different comparison, so that one does start at the top.
+    /// </summary>
+    [Test]
+    public async Task Selecting_another_entry_starts_at_its_top()
+    {
+        var state = Scrolled();
+
+        var selected = ViewerSession.SelectKey(state, state.Queue[0].Key);
+
+        await Assert.That(selected.Current!.Key).IsEqualTo(state.Queue[0].Key);
+        await Assert.That(selected.ScrollTop).IsEqualTo(0);
+    }
+
+    static SessionState Scrolled()
+    {
+        var state = Fixtures.Inline(
+            Fixtures.Patch("A.cs", 1, null, Fixtures.Long(true)),
+            Fixtures.Patch("B.cs", 2, null, Fixtures.Long(true)));
+        state = ViewerSession.SelectKey(state, state.Queue[1].Key);
+        state = ViewerSession.Apply(state, CommandKind.PageDown);
+        if (state.ScrollTop == 0)
+        {
+            throw new("The entry did not scroll, so nothing below asserts anything.");
+        }
+
+        return state;
+    }
+
+    static int VisibleRowOf(SessionState state, int entry)
+    {
+        var visible = QueueProjection.Visible(state, ScreenBuilder.BodyRows(state), out _);
+        for (var index = 0; index < visible.Count; index++)
+        {
+            if (visible[index].Kind == QueueRowKind.Entry &&
+                visible[index].EntryIndex == entry)
+            {
+                return index;
+            }
+        }
+
+        throw new($"Entry {entry} is not on screen.");
+    }
+}

src/DiffEngineViewer/ViewerSession.cs

@@ -890,6 +890,14 @@ static SessionState Select(SessionState state, int index)
             return state;
         }
 
+        // Already the entry on screen. Selecting it is what a click on it does, what a right click
+        // opening its menu does, and what a focus naming it does, and none of those asks to be
+        // taken back to the top of what is being read.
+        if (index == state.Selected)
+        {
+            return Clamp(state);
+        }
+
         return Clamp(state with
         {
             Selected = index,