fix(DatePicker): align range panel anchor and endpoint highlights

docs/controls/desktop/data-entry/date-picker/changelog.md

@@ -13,6 +13,7 @@
   - Align RangeDatePicker preview precedence with the shared range model so reverse start editing renders only the normalized live endpoints instead of retaining a third committed endpoint.
   - Keep range Hover state atomic across shared panels and preserve the established endpoint corner geometry.
   - Preserve the opposite endpoint value, input text and DateViewer selected range state when confirming one RangeDatePicker endpoint and rotating the active endpoint.
+  - Keep complete range popups anchored to the start period and suppress selected/range endpoint projection on duplicate outside cells in dual-panel range viewers.
 
 ## 2026-10-03
 

docs/controls/desktop/data-entry/date-picker/implementation.md

@@ -77,9 +77,10 @@ Pointer、键盘、滚轮与 Automation 共用面板算法，事件消费由 Pic
 ## 8. 算法与关键流程
 
 单位归一、范围视觉与导航由共享内核提供；Formatting 保留输入格式和宽度。
-终点打开按该端点与面板周期定位，起点不能通过同步改写浏览区域。
+仅有终点值时，打开按该端点与面板周期定位；起点不能通过同步改写浏览区域。
 已有完整范围重新打开时，本次确认进度从当前活动端开始重建；选择当前端点后保持弹层打开并转移到另一端，
 只有另一端也在本次编辑中确认后才执行最终确认和关闭。
+完整范围双面板打开时以 start 所在周期作为左面板锚点；active end 不能倒推一个周期来改变面板锚点。
 Today/Now/Confirm 共用有效约束，但时间来源和确认行为按原规则。
 视觉顺序与候选端点分离，范围预览不能改写真实值，反向 hover 只改变预览文本和预览高亮的显示顺序。
 选择/确认一个端点时必须结束旧 Hover 预览，避免旧 hover 在活动端翻转后被解释成新活动端的预览并覆盖 viewer 已提交选中态。

docs/controls/desktop/data-entry/date-viewer/shared-panel-design.md

@@ -222,6 +222,7 @@ Picker、Calendar 以及 DateViewer 分别拥有自己的 Semantic owner。组
 提交前重新验证有效性，避免模型生成后约束发生变化使过期 Cell 被提交。
 范围真实端点、候选端点和 hover preview 分别推导；预览中间区域与端点不污染真实选中标记。
 激活有效 Cell 时必须先清理旧 Hover preview，再让宿主提交和切换活动端，防止旧 hover 在新活动端上下文中覆盖已提交范围选中态。
+双面板范围选择中，非本面板周期的 outside cell 只作为浏览填充，不投影 selected、visual endpoint、range endpoint、range middle 或 preview 状态；同一个真实端点只能在所属面板周期内高亮一次。
 周模式按整行输出连续视觉，周号与七个日期参与同一范围；命中区域包含日期格间隙和两个面板，不只依赖 Cell PointerEntered。
 
 ### 7.2 Picker 编辑会话
@@ -241,6 +242,7 @@ Picker、Calendar 以及 DateViewer 分别拥有自己的 Semantic owner。组
 范围活动端点的显示定位使用该端点与当前面板周期，不允许开始值同步将结束面板拉回开始区域。
 RangeDatePicker 每次打开都重建本次端点确认进度：空范围从 start 到 end；单端点范围把已有端点作为固定端；
 完整范围保留提交值作为初始显示，但本次编辑仍要求从当前活动端开始重新确认两个端点。
+完整范围使用 start 所在周期作为双面板左锚点；active end 只表达当前编辑端点，不能通过倒推一个周期改变左面板月份。
 设置当前端点后，活动端、输入焦点、Popup 箭头和 DatePanel active part 必须同步翻转到另一端。
 端点翻转不得清理对向端点的值或本端刚确认值；清理行为只允许发生在显式 Clear、表单清空或既有关闭路径要求的未完整范围回滚中。
 范围日期时间的有效确认行为与用户原始 IsNeedConfirm 请求分开表示，不需要覆盖 public 属性再保存 backup 来表示内部派生状态；

src/AtomUI.Desktop.Controls/DatePicker/RangeDatePickerPresenter.cs

@@ -55,21 +55,9 @@ internal class RangeDatePickerPresenter : DatePickerPresenter
     protected override DateTime? ResolveOpenDisplayAnchor()
     {
         var active = EditSession.OpenAnchor;
-        if (active is null || PanelCount == 1 || ActiveRangePart == DateRangeActivePart.Start)
+        if (active is null || PanelCount == 1)
             return active;
-        try
-        {
-            return PickerMode switch
-            {
-                DatePickerMode.Month or DatePickerMode.Quarter => active.Value.AddYears(-1),
-                DatePickerMode.Year => new DateTime(Math.Max(1, active.Value.Year / 10 * 10 - 10), 1, 1),
-                _ => active.Value.AddMonths(-1)
-            };
-        }
-        catch (ArgumentOutOfRangeException)
-        {
-            return active;
-        }
+        return EditSession.Draft.Start ?? active;
     }
 
     protected override void ApplyDraft(DateViewerRange draft)

src/AtomUI.Desktop.Controls/DateViewer/Internal/DatePanelAlgorithms.cs

@@ -237,12 +237,13 @@ internal static class DatePanelAlgorithms
         bool Matches(DateTime? target) => value is { } candidate && target is { } selection &&
             Normalize(candidate, comparisonUnit, firstDay) == Normalize(selection, comparisonUnit, firstDay);
         var selectableWeek = kind != DateViewerCellType.Week || input.SelectionUnit == DateViewerSelectionUnit.Week;
-        var rangeGrid = input.SelectionUnit == comparisonUnit && input.Range is not null;
+        var rangeVisualEligible = IsRangeVisualEligible(input, inView);
+        var rangeGrid = rangeVisualEligible && input.SelectionUnit == comparisonUnit && input.Range is not null;
         var state = rangeGrid && value is { } rangeValue
             ? GetRangeState(input, rangeValue, firstDay)
             : default;
         var selected = !state.HasPreview && !disabled && selectableWeek && (Matches(input.SelectedDate) ||
-            input.Range is { } range && (Matches(range.Start) || Matches(range.End)));
+            rangeVisualEligible && input.Range is { } range && (Matches(range.Start) || Matches(range.End)));
         var hovered = !disabled && selectableWeek && Matches(input.HoveredValue);
         var visualEndpoint = state.HasPreview ? state.PreviewStart || state.PreviewEnd : selected;
         var weekVisual = input.SelectionUnit == DateViewerSelectionUnit.Week && input.PanelKind == DateViewerPanelKind.Date &&
@@ -266,6 +267,9 @@ internal static class DatePanelAlgorithms
         };
     }
 
+    private static bool IsRangeVisualEligible(DatePanelInput input, bool isInView) =>
+        input is not { IsRangeSelection: true, PanelCount: > 1 } || isInView;
+
     private static DateViewerSelectionUnit GetCellUnit(DateViewerCellType kind) => kind switch
     {
         DateViewerCellType.Date => DateViewerSelectionUnit.Date,

tests/AtomUI.Desktop.Controls.Tests/DatePicker/DatePickerViewerBehaviorTests.cs

@@ -144,7 +144,7 @@ public class DatePickerViewerBehaviorTests
     }
 
     [Fact]
-    public void Range_End_Open_Uses_The_Previous_Dual_Panel_Period_And_Keeps_The_Host_As_Value_Owner()
+    public void Range_End_Open_Uses_The_Start_Dual_Panel_Period_And_Keeps_The_Host_As_Value_Owner()
     {
         var start = new DateTime(2026, 1, 12);
         var end = new DateTime(2026, 2, 11);
@@ -164,7 +164,7 @@ public class DatePickerViewerBehaviorTests
             var presenter = picker.PickerPresenter.ShouldBeAssignableTo<RangeDatePickerPresenter>()!;
             var viewer = presenter.GetVisualDescendants().OfType<RangeDateViewer>().Single();
 
-            viewer.PanelSession.Input.DisplayDate.ShouldBe(end.AddMonths(-1));
+            viewer.PanelSession.Input.DisplayDate.ShouldBe(start);
             viewer.PanelSession.Input.Range.ShouldBe(new DateViewerRange(start, end));
             viewer.PanelSession.Input.PanelCount.ShouldBe(2);
             viewer.Bounds.Width.ShouldBe(540, 0.5);
@@ -178,6 +178,35 @@ public class DatePickerViewerBehaviorTests
         });
     }
 
+    [Fact]
+    public void Range_End_Open_Keeps_The_Start_Period_As_The_Left_Dual_Panel_When_The_Range_Is_In_One_Month()
+    {
+        var start = new DateTime(2026, 7, 6);
+        var end = new DateTime(2026, 7, 12);
+        var picker = new RangeDatePicker
+        {
+            PickerDisplayDate = new DateTime(2027, 9, 23),
+            RangeStartSelectedDate = start,
+            RangeEndSelectedDate = end
+        };
+        picker.RangeActivatedPart = RangeActivatedPart.End;
+
+        ShowPicker(picker, () =>
+        {
+            picker.IsPickerOpen = true;
+            Drain();
+
+            var presenter = picker.PickerPresenter.ShouldBeAssignableTo<RangeDatePickerPresenter>()!;
+            var viewer = presenter.GetVisualDescendants().OfType<RangeDateViewer>().Single();
+
+            viewer.PanelSession.Input.DisplayDate.ShouldBe(start);
+            viewer.PanelSession.Models[0].Anchor.ShouldBe(new DateTime(2026, 7, 1));
+            viewer.PanelSession.Models[1].Anchor.ShouldBe(new DateTime(2026, 8, 1));
+            viewer.PanelSession.Input.Range.ShouldBe(new DateViewerRange(start, end));
+            viewer.PanelSession.Input.ActiveRangePart.ShouldBe(DateRangeActivePart.End);
+        });
+    }
+
     [Fact]
     public void Range_Partial_Confirmation_Stays_Open_And_Final_Confirmation_Orders_Dates()
     {

tests/AtomUI.Desktop.Controls.Tests/DateViewer/DateViewerTests.cs

@@ -84,6 +84,54 @@ public class DateViewerTests
         finally { window.Close(); Dispatcher.UIThread.RunJobs(); }
     }
 
+    [Fact]
+    public void Range_Dual_Panel_Does_Not_Select_Duplicate_Outside_Endpoint_Cells()
+    {
+        var start = new DateTime(2026, 9, 28);
+        var end = new DateTime(2026, 10, 4);
+        var viewer = new RangeDateViewer
+        {
+            DisplayDate = new DateTime(2026, 9, 1),
+            Value = new DateViewerRange(start, end)
+        };
+        var window = new Avalonia.Controls.Window { Width = 600, Height = 360, Content = viewer };
+
+        try
+        {
+            window.Show();
+            Dispatcher.UIThread.RunJobs();
+
+            var cells = viewer.GetVisualDescendants().OfType<DateViewerCell>().ToArray();
+            var visibleStart = cells.Single(cell => cell.Model?.Value == start && cell.Model.IsInView);
+            var visibleEnd = cells.Single(cell => cell.Model?.Value == end && cell.Model.IsInView);
+            var duplicateStart = cells.Single(cell => cell.Model?.Value == start && cell.Model.IsInView == false);
+            var duplicateEnd = cells.Single(cell => cell.Model?.Value == end && cell.Model.IsInView == false);
+
+            visibleStart.Model!.IsSelected.ShouldBeTrue();
+            visibleStart.Model.IsVisualEndpoint.ShouldBeTrue();
+            visibleStart.Model.IsRangeStart.ShouldBeTrue();
+            visibleEnd.Model!.IsSelected.ShouldBeTrue();
+            visibleEnd.Model.IsVisualEndpoint.ShouldBeTrue();
+            visibleEnd.Model.IsRangeEnd.ShouldBeTrue();
+
+            duplicateStart.Model!.IsSelected.ShouldBeFalse();
+            duplicateStart.Model.IsVisualEndpoint.ShouldBeFalse();
+            duplicateStart.Model.IsRangeStart.ShouldBeFalse();
+            duplicateStart.Model.IsRangeMiddle.ShouldBeFalse();
+            duplicateStart.Model.IsRangeEnd.ShouldBeFalse();
+            duplicateEnd.Model!.IsSelected.ShouldBeFalse();
+            duplicateEnd.Model.IsVisualEndpoint.ShouldBeFalse();
+            duplicateEnd.Model.IsRangeStart.ShouldBeFalse();
+            duplicateEnd.Model.IsRangeMiddle.ShouldBeFalse();
+            duplicateEnd.Model.IsRangeEnd.ShouldBeFalse();
+        }
+        finally
+        {
+            window.Close();
+            Dispatcher.UIThread.RunJobs();
+        }
+    }
+
     [Fact]
     public void Header_Commands_Browse_And_Same_Value_Selection_Notifies_Once_Per_Activation()
     {