Added some NeedDisplay unit tess

UnitTests/View/NeedsDisplayTests.cs

@@ -0,0 +1,51 @@
+#nullable enable
+using System.Text;
+using Xunit.Abstractions;
+
+namespace Terminal.Gui.ViewTests;
+
+[Trait("Category","Output")]
+public class NeedsDisplayTests (ITestOutputHelper output)
+{
+    [Fact]
+    public void NeedsDisplay_False_If_Width_Height_Zero ()
+    {
+        View view = new () { Width = 0, Height = 0};
+        view.BeginInit();
+        view.EndInit();
+        Assert.False (view.NeedsDisplay);
+        Assert.False (view.SubViewNeedsDisplay);
+    }
+
+
+    [Fact]
+    public void NeedsDisplay_True_Initially_If_Width_Height_Not_Zero ()
+    {
+        View superView = new () { Width = 1, Height = 1};
+        View view1 = new () { Width = 1, Height = 1 };
+        View view2 = new () { Width = 1, Height = 1 };
+
+        superView.Add(view1, view2);
+        superView.BeginInit ();
+        superView.EndInit ();
+
+        Assert.True (superView.NeedsDisplay);
+        Assert.True (superView.SubViewNeedsDisplay);
+        Assert.True (view1.NeedsDisplay);
+        Assert.True (view2.NeedsDisplay);
+
+        superView.Draw ();
+
+        Assert.False (superView.NeedsDisplay);
+        Assert.False (superView.SubViewNeedsDisplay);
+        Assert.False (view1.NeedsDisplay);
+        Assert.False (view2.NeedsDisplay);
+
+        superView.SetNeedsDisplay();
+
+        Assert.True (superView.NeedsDisplay);
+        Assert.True (superView.SubViewNeedsDisplay);
+        Assert.True (view1.NeedsDisplay);
+        Assert.True (view2.NeedsDisplay);
+    }
+}