Teach the page and the shell to put a reattached view back together

The page's socket now retries itself forever with backoff — a dropped
socket is an ordinary event on a phone, not the end of the terminal's
life — and createSession is idempotent, so a replay landing on a pane
that survived changes nothing. A replay creating a pane that did not
survive writes one dim line saying the earlier output stayed on the
host, because that is the truth about a reloaded page's scrollback.

The shell answers RendererReattached with the two things only it owns:
the font size, and which tab is active.

src/DodoSSH.Client.Shell/ViewModels/MainWindowViewModel.cs

@@ -525,6 +525,7 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
         // list. Detached in DisposeAsync, which is the only point either of them ends.
         this.workspace.SessionEnded += OnWorkspaceSessionEnded;
         this.workspace.FontSizeStepRequested += OnFontSizeStepRequested;
+        this.workspace.RendererReattached += OnRendererReattached;
 
         settings = new ClientSettingsStore(paths);
 
@@ -635,6 +636,31 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
     private void OnFontSizeStepRequested(object? sender, TerminalFontSizeStepEventArgs e) =>
         Dispatcher.UIThread.Post(() => StepTerminalFontSize(e.Step));
 
+    /// <remarks>
+    /// <para>
+    /// Marshalled for the same reason as <see cref="OnFontSizeStepRequested"/>: this arrives on the data
+    /// plane's socket-accept thread, and both properties it reads here — <see cref="TerminalFontSize"/> and
+    /// <see cref="SelectedTab"/> — are bound to by the interface.
+    /// </para>
+    /// <para>
+    /// <see cref="TerminalWorkspace.RendererReattached"/> fires once the workspace has replayed what it
+    /// owns — the live sessions. Font size and the choice of active tab are not the workspace's to know;
+    /// they live here, so this is the other half of putting a reattached page back the way it was. The size
+    /// is sent exactly as <see cref="TellRendererTheFontSizeAsync"/> sends it at startup, because nothing
+    /// has changed — the page has merely forgotten, and this is only a reminder.
+    /// </para>
+    /// </remarks>
+    private void OnRendererReattached(object? sender, EventArgs e) =>
+        Dispatcher.UIThread.Post(() =>
+        {
+            _ = workspace.SetFontSizeAsync(TerminalFontSize, CancellationToken.None).AsTask();
+
+            if (SelectedTab is { } tab)
+            {
+                _ = workspace.ActivateSessionAsync(tab.SessionId, CancellationToken.None).AsTask();
+            }
+        });
+
     [ObservableProperty]
     private ShellState state = ShellState.Starting;
 
@@ -3051,6 +3077,7 @@ internal sealed partial class MainWindowViewModel : ObservableObject, IAsyncDisp
 
         workspace.SessionEnded -= OnWorkspaceSessionEnded;
         workspace.FontSizeStepRequested -= OnFontSizeStepRequested;
+        workspace.RendererReattached -= OnRendererReattached;
         transfers.PropertyChanged -= OnTransfersPropertyChanged;
 
         // Stopped here rather than left to the process exiting with it: the loop holds no vault key and