Bug explanation

Based on #2951, see also PR #2954

I dig myself a bit into the code and came up with two suggestions:

In a normal (non-DataGrid), editable ComboBox's ControlTemplate there is a definition of CaretBrush to be equal to BorderBrush (?). Makes no sense to me, but probably harms nothing. Wouldn't that be better like

CaretBrush="{TemplateBinding Foreground}"

Along this analogy, for the DataGrid version MaterialDesignDataGridComboBoxEditableTextBox instead of asking the style to do an obscure crawl up into the visual tree for a property (as was in the original PR):

<Setter Property="CaretBrush" Value="{Binding RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}, Path=(TextElement.Foreground)}" />

we could just add MaterialDesignDataGridComboBoxEditableTemplate ControlTemplate's TextBox x:Name="PART_EditableTextBox" the same:

CaretBrush="{TemplateBinding Foreground}"

The latter seems more neat to me.

Version

Latest master