Fixes #3124. 0001 for year in DateField. (#3131)

* Prefix private fields with underscore.

* Rename to SetInitialProperties.

* Ensures clear all selection if there isn't selecting.

* Fix format and replace duplicate key bindings for DeleteCharLeft.

* Test the new key binding for DeleteCharLeft.

* Fix formating when typing with selection which may broken the format.

* Fix copy/paste.

* Test data from 01/01/0001 to 12/31/9999.

UnitTests/Views/DateFieldTests.cs

@@ -57,6 +57,31 @@ namespace Terminal.Gui.ViewsTests {
 			Assert.Equal (8, df.CursorPosition);
 		}
 
+		[Fact]
+		public void CursorPosition_Min_Is_Always_One_Max_Is_Always_Max_Format_After_Selection ()
+		{
+			var df = new DateField ();
+			// Start selection
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.CursorLeft | KeyCode.ShiftMask)));
+			Assert.Equal (1, df.SelectedStart);
+			Assert.Equal (1, df.SelectedLength);
+			Assert.Equal (0, df.CursorPosition);
+			// Without selection
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.CursorLeft)));
+			Assert.Equal (-1, df.SelectedStart);
+			Assert.Equal (0, df.SelectedLength);
+			Assert.Equal (1, df.CursorPosition);
+			df.CursorPosition = 10;
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.CursorRight | KeyCode.ShiftMask)));
+			Assert.Equal (10, df.SelectedStart);
+			Assert.Equal (1, df.SelectedLength);
+			Assert.Equal (11, df.CursorPosition);
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.CursorRight)));
+			Assert.Equal (-1, df.SelectedStart);
+			Assert.Equal (0, df.SelectedLength);
+			Assert.Equal (10, df.CursorPosition);
+		}
+
 		[Fact]
 		public void KeyBindings_Command ()
 		{
@@ -97,6 +122,63 @@ namespace Terminal.Gui.ViewsTests {
 			df.ReadOnly = false;
 			Assert.True (df.NewKeyDownEvent (new (KeyCode.D1)));
 			Assert.Equal (" 12/02/1971", df.Text);
+			Assert.Equal (2, df.CursorPosition);
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.D | KeyCode.AltMask)));
+			Assert.Equal (" 10/02/1971", df.Text);
+			CultureInfo.CurrentCulture = cultureBackup;
+		}
+
+		[Fact]
+		public void Typing_With_Selection_Normalize_Format ()
+		{
+			CultureInfo cultureBackup = CultureInfo.CurrentCulture;
+			CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
+			DateField df = new DateField (DateTime.Parse ("12/12/1971"));
+			// Start selection at before the first separator /
+			df.CursorPosition = 2;
+			// Now select the separator /
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.CursorRight | KeyCode.ShiftMask)));
+			Assert.Equal (2, df.SelectedStart);
+			Assert.Equal (1, df.SelectedLength);
+			Assert.Equal (3, df.CursorPosition);
+			// Type 3 over the separator
+			Assert.True (df.NewKeyDownEvent (new (KeyCode.D3)));
+			// The format was normalized and replaced again with /
+			Assert.Equal (" 12/12/1971", df.Text);
+			Assert.Equal (4, df.CursorPosition);
+			CultureInfo.CurrentCulture = cultureBackup;
+		}
+
+		[Fact, AutoInitShutdown]
+		public void Copy_Paste ()
+		{
+			CultureInfo cultureBackup = CultureInfo.CurrentCulture;
+			CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
+			DateField df1 = new DateField (DateTime.Parse ("12/12/1971"));
+			DateField df2 = new DateField (DateTime.Parse ("12/31/2023"));
+			// Select all text
+			Assert.True (df2.NewKeyDownEvent (new (KeyCode.End | KeyCode.ShiftMask)));
+			Assert.Equal (1, df2.SelectedStart);
+			Assert.Equal (10, df2.SelectedLength);
+			Assert.Equal (11, df2.CursorPosition);
+			// Copy from df2
+			Assert.True (df2.NewKeyDownEvent (new (KeyCode.C | KeyCode.CtrlMask)));
+			// Paste into df1
+			Assert.True (df1.NewKeyDownEvent (new (KeyCode.V | KeyCode.CtrlMask)));
+			Assert.Equal (" 12/31/2023", df1.Text);
+			Assert.Equal (11, df1.CursorPosition);
+			CultureInfo.CurrentCulture = cultureBackup;
+		}
+
+		[Fact]
+		public void Date_Start_From_01_01_0001_And_End_At_12_31_9999 ()
+		{
+			CultureInfo cultureBackup = CultureInfo.CurrentCulture;
+			CultureInfo.CurrentCulture = CultureInfo.InvariantCulture;
+			DateField df = new DateField (DateTime.Parse ("01/01/0001"));
+			Assert.Equal (" 01/01/0001", df.Text);
+			df.Date = DateTime.Parse ("12/31/9999");
+			Assert.Equal (" 12/31/9999", df.Text);
 			CultureInfo.CurrentCulture = cultureBackup;
 		}
 	}