Fix Placeholder not visible (#21024)

* Add default PlaceholderForeground to AutoCompleteBox, CalendarDatePicker and NumericUpDown.

* Add PlaceholderText template binding for simple theme ComboBox PART_EditableTextBox.

src/Avalonia.Themes.Fluent/Controls/AutoCompleteBox.xaml

@@ -25,6 +25,7 @@
       <Setter Property="VerticalAlignment" Value="Top" />
       <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
       <Setter Property="Background" Value="{DynamicResource TextControlBackground}" />
+      <Setter Property="PlaceholderForeground" Value="{DynamicResource TextControlPlaceholderForeground}" />
       <Setter Property="BorderBrush" Value="{DynamicResource TextControlBorderBrush}" />
       <Setter Property="BorderThickness" Value="{DynamicResource TextControlBorderThemeThickness}" />
       <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />

src/Avalonia.Themes.Fluent/Controls/CalendarDatePicker.xaml

@@ -63,6 +63,7 @@
   <ControlTheme x:Key="{x:Type CalendarDatePicker}" TargetType="CalendarDatePicker">
     <Setter Property="Background" Value="{DynamicResource CalendarDatePickerBackground}" />
     <Setter Property="Foreground" Value="{DynamicResource CalendarDatePickerForeground}" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource TextControlPlaceholderForeground}" />
     <Setter Property="BorderBrush" Value="{DynamicResource CalendarDatePickerBorderBrush}" />
     <Setter Property="BorderThickness" Value="{DynamicResource CalendarDatePickerBorderThemeThickness}" />
     <Setter Property="CornerRadius" Value="{DynamicResource ControlCornerRadius}" />

src/Avalonia.Themes.Fluent/Controls/NumericUpDown.xaml

@@ -52,6 +52,7 @@
   <ControlTheme x:Key="{x:Type NumericUpDown}" TargetType="NumericUpDown">
     <Setter Property="Foreground" Value="{DynamicResource TextControlForeground}" />
     <Setter Property="Background" Value="{DynamicResource TextControlBackground}" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource TextControlPlaceholderForeground}" />
     <Setter Property="BorderThickness" Value="{DynamicResource TextControlBorderThemeThickness}" />
     <Setter Property="BorderBrush" Value="{DynamicResource TextControlBorderBrush}" />
     <Setter Property="MinHeight" Value="{DynamicResource TextControlThemeMinHeight}" />

src/Avalonia.Themes.Simple/Controls/AutoCompleteBox.xaml

@@ -4,6 +4,7 @@
   <ControlTheme x:Key="{x:Type AutoCompleteBox}"
                 TargetType="AutoCompleteBox">
     <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource ThemeForegroundLowBrush}" />
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
     <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
     <Setter Property="Padding" Value="4" />

src/Avalonia.Themes.Simple/Controls/CalendarDatePicker.xaml

@@ -12,6 +12,7 @@
   <ControlTheme x:Key="{x:Type CalendarDatePicker}"
                 TargetType="CalendarDatePicker">
     <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource ThemeForegroundLowBrush}" />
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
     <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
     <Setter Property="Padding" Value="4" />

src/Avalonia.Themes.Simple/Controls/ComboBox.xaml

@@ -50,6 +50,7 @@
                      Foreground="{TemplateBinding Foreground}"
                      Background="Transparent"
                      Text="{TemplateBinding Text, Mode=TwoWay}"
+                     PlaceholderText="{TemplateBinding PlaceholderText}"
                      BorderThickness="0"
                      IsVisible="{TemplateBinding IsEditable}">
               <TextBox.Resources>

src/Avalonia.Themes.Simple/Controls/NumericUpDown.xaml

@@ -4,6 +4,7 @@
   <ControlTheme x:Key="{x:Type NumericUpDown}"
                 TargetType="NumericUpDown">
     <Setter Property="Background" Value="{DynamicResource ThemeBackgroundBrush}" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource ThemeForegroundLowBrush}" />
     <Setter Property="BorderBrush" Value="{DynamicResource ThemeBorderMidBrush}" />
     <Setter Property="BorderThickness" Value="{DynamicResource ThemeBorderThickness}" />
     <Setter Property="Padding" Value="4" />