Position date and time picker popups below fields

Fixes issue MaterialDesignInXAML#4050 by using Bottom popup placement for DatePicker and TimePicker templates, removing the unused custom popup callback helper, and adding UI tests that assert the popup opens below the text box by default.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Keboo

src/MaterialDesignThemes.Wpf/CustomPopupPlacementCallbackHelper.cs

@@ -120,8 +120,7 @@
                     Template="{StaticResource CalendarButtonTemplate}" />
             <Popup x:Name="PART_Popup"
                    AllowsTransparency="True"
-                   CustomPopupPlacementCallback="{x:Static wpf:CustomPopupPlacementCallbackHelper.LargePopupCallback}"
-                   Placement="Custom"
+                   Placement="Bottom"
                    PlacementTarget="{Binding ElementName=PART_TextBox}"
                    PopupAnimation="Fade"
                    StaysOpen="False" />

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.DatePicker.xaml

@@ -132,8 +132,7 @@
                     Template="{StaticResource ClockButtonTemplate}" />
             <Popup x:Name="PART_Popup"
                    AllowsTransparency="True"
-                   CustomPopupPlacementCallback="{x:Static wpf:CustomPopupPlacementCallbackHelper.LargePopupCallback}"
-                   Placement="Custom"
+                   Placement="Bottom"
                    PlacementTarget="{Binding ElementName=PART_TextBox}"
                    PopupAnimation="Fade"
                    StaysOpen="False" />

src/MaterialDesignThemes.Wpf/Themes/MaterialDesignTheme.TimePicker.xaml

@@ -333,6 +333,35 @@ public async Task DatePicker_ShouldApplyIsMouseOverTriggers_WhenHoveringCalendar
 
         recorder.Success();
     }
+
+    [Test]
+    [Description("Issue 4050")]
+    public async Task DatePicker_PopupOpensBelowTextBoxByDefault()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        var stackPanel = await LoadXaml<StackPanel>("""
+            <StackPanel Margin="40">
+              <DatePicker Width="200" />
+            </StackPanel>
+            """);
+        var datePicker = await stackPanel.GetElement<DatePicker>("/DatePicker");
+        var datePickerTextBox = await datePicker.GetElement<DatePickerTextBox>("/DatePickerTextBox");
+        var button = await datePicker.GetElement<Button>("PART_Button");
+        var popup = await datePicker.GetElement<Popup>("PART_Popup");
+
+        await button.LeftClick();
+        await Wait.For(async () => await popup.GetIsOpen());
+
+        Rect? textBoxCoordinates = await datePickerTextBox.GetCoordinates();
+        Rect? popupCoordinates = await popup.GetCoordinates();
+
+        await Assert.That(textBoxCoordinates).IsNotNull();
+        await Assert.That(popupCoordinates).IsNotNull();
+        await Assert.That(popupCoordinates.Value.Top).IsGreaterThanOrEqualTo(textBoxCoordinates.Value.Bottom - 1);
+
+        recorder.Success();
+    }
 }
 
 public class FutureDateValidationRule : ValidationRule

tests/MaterialDesignThemes.UITests/WPF/DatePickers/DatePickerTests.cs

@@ -642,6 +642,35 @@ public async Task TimePicker_ShouldApplyIsMouseOverTriggers_WhenHoveringTimeButt
         recorder.Success();
     }
 
+    [Test]
+    [Description("Issue 4050")]
+    public async Task TimePicker_PopupOpensBelowTextBoxByDefault()
+    {
+        await using var recorder = new TestRecorder(App);
+
+        var stackPanel = await LoadXaml<StackPanel>("""
+            <StackPanel Margin="40">
+              <materialDesign:TimePicker Width="200" />
+            </StackPanel>
+            """);
+        var timePicker = await stackPanel.GetElement<TimePicker>("/TimePicker");
+        var timePickerTextBox = await timePicker.GetElement<TimePickerTextBox>("/TimePickerTextBox");
+        var button = await timePicker.GetElement<Button>("PART_Button");
+        var popup = await timePicker.GetElement<Popup>("PART_Popup");
+
+        await button.LeftClick();
+        await Wait.For(async () => await popup.GetIsOpen());
+
+        Rect? textBoxCoordinates = await timePickerTextBox.GetCoordinates();
+        Rect? popupCoordinates = await popup.GetCoordinates();
+
+        await Assert.That(textBoxCoordinates).IsNotNull();
+        await Assert.That(popupCoordinates).IsNotNull();
+        await Assert.That(popupCoordinates.Value.Top).IsGreaterThanOrEqualTo(textBoxCoordinates.Value.Bottom - 1);
+
+        recorder.Success();
+    }
+
     [Test]
     [Description("Issue 3650")]
     public async Task TimePicker_MovesFocusToPrevious_WhenShiftAndTabIsPressed()