fix(TextBox): initialize command states in constructor (#21859)

Author: zrt2399

src/Avalonia.Controls/TextBox.cs

@@ -431,6 +431,7 @@ public TextBox()
             _undoRedoHelper = new UndoRedoHelper<UndoRedoState>(this);
             _selectedTextChangesMadeSinceLastUndoSnapshot = 0;
             _hasDoneSnapshotOnce = false;
+            UpdateCommandStates();
             UpdatePseudoclasses();
         }
 

tests/Avalonia.Controls.UnitTests/TextBoxTests.cs

@@ -1549,6 +1549,19 @@ public void Setting_IsUndoEnabled_To_False_Clears_Undo_Redo()
             }
         }
 
+        [Fact]
+        public void Empty_TextBox_Initializes_Clipboard_Command_States()
+        {
+            using (UnitTestApplication.Start(Services))
+            {
+                var tb = new TextBox();
+
+                Assert.False(tb.CanCopy);
+                Assert.False(tb.CanCut);
+                Assert.True(tb.CanPaste);
+            }
+        }
+
         [Fact]
         public void Command_States_Update_When_ReadOnly_And_PasswordChar_Change()
         {