Здесь было несколько проблем.Каждый элемент, который я изменил в XAML, комментируется.
Я также удалил триггеры IsMouseOver из стилей переключателей и текстовых полей и перевел их в стиль ComboBox.Теперь он устанавливает фон для всего поля со списком, когда мышь находится над ComboBox.Два элемента управления в шаблоне ComboBox теперь имеют Background="{TemplateBinding Background}", поэтому они будут использовать цвет фона, установленный этим триггером.
Я восстановил IsHitTestVisible в текстовом поле, но использовал TemplateBinding, чтобы связать его со свойством IsEditable ComboBox.Это даст вам правильный указатель мыши над текстовым полем.
<!--
Better to define this in one place.
I'd do the same with the border color that you use everywhere.
-->
<SolidColorBrush x:Key="ComboBackgroundBrush" Color="#EAEAEA" />
<Style x:Key="ComboBoxTextBoxStyle" TargetType="{x:Type TextBox}">
<!--
Must set this in a setter, not an an attribute on the control instance.
The attribute you had will override anything the style does. This is part of
"dependency property value precedence".
-->
<Setter Property="Background" Value="{StaticResource ComboBackgroundBrush}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type TextBox}">
<Grid>
<Border CornerRadius="2,0,0,2"
BorderThickness="1,1,0,1"
Background="{TemplateBinding Background}"
BorderBrush="#A0A1A2"
>
<ScrollViewer x:Name="PART_ContentHost"/>
</Border>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ComboBoxButtonStyle" TargetType="{x:Type ToggleButton}">
<Setter Property="Background" Value="{StaticResource ComboBackgroundBrush}" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ToggleButton}">
<!--
Needs {TemplateBinding Background} so it uses whatever background brush
the control has at any given moment.
-->
<Border Background="{TemplateBinding Background}"
x:Name="border"
CornerRadius="0,2,2,0"
BorderThickness="0,1,1,1"
BorderBrush="#A0A1A2">
<ContentPresenter />
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="ComboBoxOverlayToggleButtonStyle" TargetType="ToggleButton">
<Setter Property="Focusable" Value="False" />
<Setter Property="ClickMode" Value="Press" />
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="ToggleButton">
<Grid Background="Transparent" />
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
<Style x:Key="RoundComboBox" TargetType="{x:Type ComboBox}" BasedOn="{StaticResource BorderStyle}">
<Setter Property="HorizontalContentAlignment" Value="Left"/>
<Setter Property="VerticalContentAlignment" Value="Center"/>
<Setter Property="FontSize" Value="14px"/>
<Setter Property="IsReadOnly" Value="True"/>
<Setter Property="Height" Value="25"/>
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type ComboBox}">
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition MaxWidth="18"/>
</Grid.ColumnDefinitions>
<ToggleButton
Grid.Column="0"
Style="{StaticResource ComboBoxOverlayToggleButtonStyle}"
IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
/>
<!--
Two problems:
1. IsHitTestVisible="False" prevented it from getting any mouse messages.
2. Background attribute was overriding anything the Style did,
so even if the trigger had fired, its setter would have failed.
Also, Height="{TemplateBinding Height}" is unnecessary. It'll size to its parent Grid.
And BorderBrush="#A0A1A2" should probably be in the Style
-->
<TextBox Name="PART_EditableTextBox"
Padding="0,0,0,0"
BorderBrush="#A0A1A2"
Style="{StaticResource ComboBoxTextBoxStyle}"
Background="{TemplateBinding Background}"
IsHitTestVisible="{TemplateBinding IsEditable}"
/>
<ToggleButton Grid.Column="1"
Height="{TemplateBinding Height}"
Style="{StaticResource ComboBoxButtonStyle}"
Focusable="False"
IsChecked="{Binding Path=IsDropDownOpen, Mode=TwoWay, RelativeSource={RelativeSource TemplatedParent}}"
ClickMode="Press"
Background="{TemplateBinding Background}"
>
<Path Grid.Column="1"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Data="M 0 0 L 4 4 L 8 0 Z"
Fill="Black" />
</ToggleButton>
<ContentPresenter Grid.Column="0"
Name="ContentSite"
Content="{TemplateBinding SelectionBoxItem}"
ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
VerticalAlignment="Center"
HorizontalAlignment="Left"
Margin="6,0,0,0"
/>
<Popup Grid.Column="0"
Name="Popup"
Placement="Bottom"
IsOpen="{TemplateBinding IsDropDownOpen}"
AllowsTransparency="True"
Focusable="False"
PopupAnimation="Slide">
<Grid Name="DropDown"
SnapsToDevicePixels="True"
MinWidth="{TemplateBinding ActualWidth}"
MaxHeight="{TemplateBinding MaxDropDownHeight}">
<Border x:Name="DropDownBorder"
BorderThickness="1"
CornerRadius="2"
Background="White"
BorderBrush="#A0A1A2"/>
<ScrollViewer SnapsToDevicePixels="True">
<StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
</ScrollViewer>
</Grid>
</Popup>
</Grid>
</ControlTemplate>
</Setter.Value>
</Setter>
<Style.Triggers>
<Trigger Property="IsMouseOver" Value="true">
<Setter Property="Background" Value="Red" />
</Trigger>
</Style.Triggers>
</Style>