Merge pull request #4369 from rumkit/fixes/4359-missing-combobox-placeholder

Added ComboBox placeholder to default theme template

src/Avalonia.Themes.Default/ComboBox.xaml

@@ -1,4 +1,5 @@
-<Styles xmlns="https://github.com/avaloniaui">
+<Styles xmlns="https://github.com/avaloniaui"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <Design.PreviewWith>
     <Border Padding="20">
       <StackPanel Spacing="10">
@@ -24,6 +25,7 @@
     <Setter Property="VerticalContentAlignment" Value="Center" />
     <Setter Property="Padding" Value="4" />
     <Setter Property="MinHeight" Value="20" />
+    <Setter Property="PlaceholderForeground" Value="{DynamicResource ComboBoxPlaceHolderForeground}" />
     <Setter Property="Template">
       <ControlTemplate>
         <Border Name="border"
@@ -31,6 +33,13 @@
                 BorderBrush="{TemplateBinding BorderBrush}"
                 BorderThickness="{TemplateBinding BorderThickness}">
           <Grid ColumnDefinitions="*,Auto">
+            <TextBlock Name="PlaceholderTextBlock"
+                       Grid.Column="0"
+                       HorizontalAlignment="{TemplateBinding HorizontalContentAlignment}"
+                       VerticalAlignment="{TemplateBinding VerticalContentAlignment}"
+                       Margin="{TemplateBinding Padding}"
+                       Text="{TemplateBinding PlaceholderText}"
+                       IsVisible="{TemplateBinding SelectionBoxItem, Converter={x:Static ObjectConverters.IsNull}}" />
             <ContentControl Content="{TemplateBinding SelectionBoxItem}"
                             ContentTemplate="{TemplateBinding ItemTemplate}"
                             Margin="{TemplateBinding Padding}"