Desktop: build only the host cards near the view, with a virtualising wrap panel for the board

tests/DodoSSH.Client.App.Layout.Tests/HostGridTests.cs

@@ -567,6 +567,69 @@ public sealed class HostGridTests : IAsyncLifetime
         });
     }
 
+    /// <remarks>
+    /// <para>
+    /// The board was a plain wrap panel, which built every card on it whether it was in view or not — some
+    /// twenty-five controls a card, so three hundred hosts took seconds to open and to redraw after a filter
+    /// was cleared. It builds the rows near the view now; see <c>VirtualizingWrapPanel</c>.
+    /// </para>
+    /// <para>
+    /// And the arrows still go everywhere, which is the half a virtualising panel can quietly break: Down
+    /// steps a whole row, and End reaches the last card although nothing had built it.
+    /// </para>
+    /// </remarks>
+    [Fact]
+    public async Task ADeepBoard_BuildsTheCardsInView_AndTheArrowsStillReachTheRest()
+    {
+        for (var host = 0; host < 120; host++)
+        {
+            vault.NewHostCommand.Execute(null);
+            vault.EditorLabel = $"zz-host-{host:000}";
+            vault.EditorHostname = $"zz-host-{host:000}.internal";
+            vault.EditorUsername = "deploy";
+
+            await vault.SaveHostCommand.ExecuteAsync(null);
+        }
+
+        await vault.LoadAsync(Token);
+
+        // Nothing selected, so the board opens at the top: a selected card is scrolled into view, and the last
+        // host saved is the one the editor left selected.
+        vault.SelectedHost = null;
+
+        await OnTheBoardAsync((screen, window) =>
+        {
+            var built = screen.GetVisualDescendants().OfType<ListBoxItem>().Count(card => card.IsVisible);
+            var total = vault.HostBoardOrder.Count();
+
+            total.ShouldBe(122);
+            built.ShouldBeGreaterThan(0);
+            built.ShouldBeLessThan(total / 2, "only the rows near the view are built");
+
+            var order = vault.HostBoardOrder.ToList();
+            var first = order[0];
+
+
+            Click(CardFor(screen, first), window);
+            vault.SelectedHost.ShouldBeSameAs(first);
+
+            var columns = CardFor(screen, first).GetVisualParent().ShouldBeOfType<VirtualizingWrapPanel>()
+                .GetVisualChildren().OfType<ListBoxItem>()
+                .Where(card => card.IsVisible)
+                .Count(card => Math.Abs(card.Bounds.Top - CardFor(screen, first).Bounds.Top) < 0.5);
+
+            window.KeyPressQwerty(PhysicalKey.ArrowDown, RawInputModifiers.None);
+            Dispatcher.UIThread.RunJobs();
+
+            vault.SelectedHost.ShouldBeSameAs(order[columns], "Down steps one whole row of cards");
+
+            window.KeyPressQwerty(PhysicalKey.End, RawInputModifiers.None);
+            Dispatcher.UIThread.RunJobs();
+
+            vault.SelectedHost.ShouldBeSameAs(order[^1], "the last card, built on the way there");
+        });
+    }
+
     // ---- Helpers ----
 
     private static void RightClick(Visual card, Visual window)