Restore the window when a snapshot asks for it

src/DiffEngineViewer.Windows.Tests/ViewerFormRaiseTests.cs

@@ -0,0 +1,50 @@
+/// <summary>
+/// Bringing the window up for a snapshot that has just arrived. The queue owner asks for this over
+/// the socket, and it is the only thing that puts a new snapshot in front of anyone.
+/// </summary>
+[NotInParallel]
+[TUnit.Core.Executors.STAThreadExecutor]
+public class ViewerFormRaiseTests
+{
+    /// <summary>
+    /// BringToFront and Activate leave a minimised window minimised: the taskbar button flashes
+    /// and nothing else happens. So a viewer that had been minimised was never actually shown the
+    /// snapshot, and the queue filled up out of sight.
+    /// </summary>
+    [Test]
+    public async Task Restores_a_minimised_window()
+    {
+        using var form = new ViewerForm("title", 800, 600);
+        form.WindowState = FormWindowState.Minimized;
+
+        form.Raise();
+
+        await Assert.That(form.WindowState).IsEqualTo(FormWindowState.Normal);
+    }
+
+    /// <summary>
+    /// A window the reader had maximised stays maximised: it is already as visible as it gets, and
+    /// restoring it would be undoing something they chose.
+    /// </summary>
+    [Test]
+    public async Task Leaves_a_maximised_window_maximised()
+    {
+        using var form = new ViewerForm("title", 800, 600);
+        form.WindowState = FormWindowState.Maximized;
+
+        form.Raise();
+
+        await Assert.That(form.WindowState).IsEqualTo(FormWindowState.Maximized);
+    }
+
+    [Test]
+    public async Task Shows_a_hidden_window()
+    {
+        using var form = new ViewerForm("title", 800, 600);
+        form.Visible = false;
+
+        form.Raise();
+
+        await Assert.That(form.Visible).IsTrue();
+    }
+}

src/DiffEngineViewer.Windows/FormsViewerWindow.cs

@@ -89,9 +89,7 @@ public void Focus()
             return;
         }
 
-        form.Visible = true;
-        form.BringToFront();
-        form.Activate();
+        form.Raise();
     }
 
     public bool Capture(Screen screen, int width, int height, string pngPath)

src/DiffEngineViewer.Windows/ViewerForm.cs

@@ -173,6 +173,27 @@ void ScaleChrome()
         scrollBar.Width = SystemInformation.GetVerticalScrollBarWidthForDpi(DeviceDpi);
     }
 
+    /// <summary>
+    /// Brings the window up, for a snapshot that has just arrived and wants reading.
+    /// <para>
+    /// The restore is the part that was missing. BringToFront and Activate leave a minimised
+    /// window minimised - the taskbar button flashes and nothing else happens - so a viewer that
+    /// had been minimised never showed the snapshot it was being asked to show, and the queue
+    /// filled up out of sight.
+    /// </para>
+    /// </summary>
+    public void Raise()
+    {
+        Visible = true;
+        if (WindowState == FormWindowState.Minimized)
+        {
+            WindowState = FormWindowState.Normal;
+        }
+
+        BringToFront();
+        Activate();
+    }
+
     public void Apply(Screen screen)
     {
         // ScreenBuilder allocates a fresh Screen every frame, so record equality would never hit.