Desktop: swap the host lists in whole instead of announcing every row to listeners that redo all their work per row

tests/DodoSSH.Client.App.Tests/ShellFlowTests.cs

@@ -669,6 +669,38 @@ public sealed partial class ShellFlowTests : IAsyncLifetime
         shell.StatusMessage.ShouldBe("Locked.");
     }
 
+    /// <remarks>
+    /// A reload used to empty the host list and refill it a row at a time, announcing each — and the transfers
+    /// screen refilled its own picker, announcing each of those, on every one of them. Three hundred hosts came
+    /// to some ninety thousand notifications. Counted here as announcements reaching the two lists, which is
+    /// what the transfers screen and the board both answer.
+    /// </remarks>
+    [Fact]
+    public async Task ReloadingTheHosts_AnnouncesTheListOnce_NotOncePerHost()
+    {
+        await UnlockedAsync();
+        var vault = shell.Vault!;
+
+        foreach (var label in new[] { "prod-db", "prod-web", "stage-web", "dev-box" })
+        {
+            await AddHostAsync(vault, label);
+        }
+
+        var vaultAnnouncements = 0;
+        var pickerAnnouncements = 0;
+
+        vault.Hosts.CollectionChanged += (_, _) => vaultAnnouncements++;
+        shell.Transfers.Hosts.CollectionChanged += (_, _) => pickerAnnouncements++;
+
+        await vault.LoadAsync(Token);
+
+        vault.Hosts.Count.ShouldBe(4);
+        shell.Transfers.Hosts.Count.ShouldBe(4);
+
+        vaultAnnouncements.ShouldBe(1);
+        pickerAnnouncements.ShouldBe(1);
+    }
+
     [Fact]
     public async Task AnAutomaticPass_SaysNothingWhenThereIsNothingToDo()
     {