diff --git a/src/Avalonia.Controls/Calendar/CalendarItem.cs b/src/Avalonia.Controls/Calendar/CalendarItem.cs
index 2bfe3c9be2..3d548e15b9 100644
--- a/src/Avalonia.Controls/Calendar/CalendarItem.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarItem.cs
@@ -299,6 +299,19 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc/>
+        protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
+        {
+            base.OnDetachedFromVisualTree(e);
+
+            // Reset mouse button tracking state. When the calendar popup closes
+            // (e.g. due to a programmatic window change during date selection),
+            // the PointerReleased event never fires, leaving these flags stuck.
+            // See https://github.com/AvaloniaUI/Avalonia/issues/18418
+            _isMouseLeftButtonDown = false;
+            _isMouseLeftButtonDownYearView = false;
+        }
+        
         private void SetDayTitles()
         {
             for (int childIndex = 0; childIndex < Calendar.ColumnsPerMonth; childIndex++)
diff --git a/tests/Avalonia.Controls.UnitTests/CalendarTests.cs b/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
index b5395f8f6e..5e4fe3aad1 100644
--- a/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
+++ b/tests/Avalonia.Controls.UnitTests/CalendarTests.cs
@@ -1,12 +1,16 @@
 using Xunit;
 using System;
-using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using Avalonia.Controls.Primitives;
+using Avalonia.Input;
+using Avalonia.UnitTests;
+
+#nullable enable
 
 namespace Avalonia.Controls.UnitTests
 {
-    public class CalendarTests
+    public class CalendarTests : ScopedTestBase
     {
         private static bool CompareDates(DateTime first, DateTime second)
         {
@@ -142,12 +146,12 @@ namespace Avalonia.Controls.UnitTests
         /// <summary>
         /// The days added to the SelectedDates collection.
         /// </summary>
-        private IList<object> _selectedDatesChangedAddedDays;
+        private IList<object>? _selectedDatesChangedAddedDays;
 
         /// <summary>
         /// The days removed from the SelectedDates collection.
         /// </summary>
-        private IList<object> _selectedDateChangedRemovedDays;
+        private IList<object>? _selectedDatesChangedRemovedDays;
 
         /// <summary>
         /// The number of times the SelectedDatesChanged event has been fired.
@@ -159,13 +163,13 @@ namespace Avalonia.Controls.UnitTests
         /// </summary>
         /// <param name="sender">The calendar.</param>
         /// <param name="e">Event arguments.</param>
-        private void OnSelectedDatesChanged(object sender, SelectionChangedEventArgs e)
+        private void OnSelectedDatesChanged(object? sender, SelectionChangedEventArgs e)
         {
             _selectedDatesChangedAddedDays =
                 e.AddedItems
                  .Cast<object>()
                  .ToList();
-            _selectedDateChangedRemovedDays = 
+            _selectedDatesChangedRemovedDays =
                 e.RemovedItems
                  .Cast<object>()
                  .ToList();
@@ -182,9 +186,9 @@ namespace Avalonia.Controls.UnitTests
                 _selectedDatesChangedAddedDays.Clear();
             }
 
-            if (_selectedDateChangedRemovedDays != null)
+            if (_selectedDatesChangedRemovedDays != null)
             {
-                _selectedDateChangedRemovedDays.Clear();
+                _selectedDatesChangedRemovedDays.Clear();
             }
 
             _selectedDatesChangedCount = 0;
@@ -202,8 +206,10 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 1);
             Assert.True(CompareDates(calendar.SelectedDates[0], DateTime.Today));
             Assert.True(_selectedDatesChangedCount == 1);
+            Assert.NotNull(_selectedDatesChangedAddedDays);
             Assert.True(_selectedDatesChangedAddedDays.Count == 1);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 0);
+            Assert.NotNull(_selectedDatesChangedRemovedDays);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 0);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDate = DateTime.Today;
@@ -240,8 +246,10 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 1);
             Assert.True(CompareDates(calendar.SelectedDates[0], DateTime.Today));
             Assert.True(_selectedDatesChangedCount == 1);
+            Assert.NotNull(_selectedDatesChangedAddedDays);
             Assert.True(_selectedDatesChangedAddedDays.Count == 1);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 0);
+            Assert.NotNull(_selectedDatesChangedRemovedDays);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 0);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDates.Clear();
@@ -263,7 +271,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 21);
             Assert.True(_selectedDatesChangedCount == 1);
             Assert.True(_selectedDatesChangedAddedDays.Count == 21);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 11);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 11);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDates.Add(DateTime.Today.AddDays(100));
@@ -362,5 +370,77 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Contains(earlierDate));
             Assert.True(calendar.SelectedDates.Contains(laterDate));
         }
+
+        [Fact]
+        public void CalendarItem_Should_Reset_Mouse_Down_Flag_On_Detach_From_Visual_Tree()
+        {
+            var calendar = new Calendar();
+            calendar.SelectionMode = CalendarSelectionMode.SingleDate;
+
+            var calendarItem = new CalendarItem();
+            calendarItem.Owner = calendar;
+
+            // Attach CalendarItem to a visual tree
+            var root = new TestRoot(calendarItem);
+
+            // Create a day button and simulate mouse left button down,
+            // which sets the internal _isMouseLeftButtonDown flag to true.
+            var date1 = new DateTime(2024, 1, 15);
+            var dayButton1 = new CalendarDayButton { DataContext = date1 };
+
+            var pointer = new Pointer(Pointer.GetNextFreeId(), PointerType.Mouse, true);
+            var props = new PointerPointProperties(RawInputModifiers.LeftMouseButton,
+                PointerUpdateKind.LeftButtonPressed);
+            var pressArgs = new PointerPressedEventArgs(dayButton1, pointer, root,
+                default, 0, props, KeyModifiers.None);
+
+            calendarItem.Cell_MouseLeftButtonDown(dayButton1, pressArgs);
+
+            // date1 should now be selected
+            Assert.Equal(1, calendar.SelectedDates.Count);
+            Assert.Equal(date1, calendar.SelectedDates[0]);
+
+            // Detach CalendarItem from visual tree (simulates popup closing
+            // during date selection without a PointerReleased event).
+            root.Child = null;
+
+            // Create a different day button and simulate mouse enter.
+            // Before the fix, _isMouseLeftButtonDown would still be true,
+            // causing hover to auto-select dates.
+            var date2 = new DateTime(2024, 1, 20);
+            var dayButton2 = new CalendarDayButton { DataContext = date2 };
+
+            calendarItem.Cell_MouseEntered(dayButton2, null!);
+
+            // The selected date should NOT have changed to date2,
+            // because the mouse-down flag was reset when detaching.
+            Assert.Equal(1, calendar.SelectedDates.Count);
+            Assert.Equal(date1, calendar.SelectedDates[0]);
+        }
+
+        [Fact]
+        public void CalendarItem_Should_Reset_YearView_Mouse_Down_Flag_On_Detach_From_Visual_Tree()
+        {
+            var calendar = new Calendar();
+            var calendarItem = new CalendarItem();
+            calendarItem.Owner = calendar;
+
+            // Attach CalendarItem to a visual tree
+            var root = new TestRoot(calendarItem);
+
+            // Use reflection to set the _isMouseLeftButtonDownYearView flag,
+            // since Month_CalendarButtonMouseDown is private.
+            var field = typeof(CalendarItem).GetField("_isMouseLeftButtonDownYearView",
+                System.Reflection.BindingFlags.NonPublic | System.Reflection.BindingFlags.Instance);
+            Assert.NotNull(field);
+            field!.SetValue(calendarItem, true);
+            Assert.True((bool)field.GetValue(calendarItem)!);
+
+            // Detach CalendarItem from visual tree
+            root.Child = null;
+
+            // Verify the flag was reset
+            Assert.False((bool)field.GetValue(calendarItem)!);
+        }
     }
 }