WPF Editable ComboBox HorizontalContentAlignment in ItemsControl problem

I have a combobox with iseditable property to true inside ItemsControl.

I set my combobox HorizontalContentAlignment to "Right".

The dropdown values are showing in right side.

But after selection the values shown in left side.

ComboBoxx:Name="comboBoxCustomReadOnly"

MinWidth="150"

MaxWidth="340"

HorizontalAlignment="Stretch"

FlowDirection="{BindingPath=Alignment}"

AllowDrop="True"

DisplayMemberPath="DisplayValue"

GotFocus="Field_GotFocus"

IsEnabled="{BindingAreCorrectionFieldsEnabled}"

IsReadOnly="True"

IsVisibleChanged="Field_IsVisibleChanged"

ItemsSource="{BindingPath=CustomFieldValues}"

SelectedValuePath="Key"

Style="{DynamicResourceSmallNormalTextStyle}"

TabIndex="32">

Please help me here.