Expander header is not stretch

There is no way to made full width expander header, without control template modification.

Steps to reproduce the behavior:

1. Apply fluent style
2. Put expander
3. Add header content with right alignment
4. Header is always aligned to left

          <Expander>
            <Expander.Header>
              <Grid ColumnDefinitions="*, Auto">
                <TextBlock Text="{Binding ComponentType}" />
                <Button Grid.Column="1" Padding="0">
                  <Image  Source="{StaticResource AppMinus}" Height="30" />
                </Button>
              </Grid>
            </Expander.Header>
            <ContentControl Content="{Binding}" />
          </Expander>

Expected behavior Content aligned to right

Screenshots

Actual result

Expected result:

Additional context This is probably because of following Grid in Expander toggle button template. https://github.com/AvaloniaUI/Avalonia/blob/25fe106c235ebb5c6260af5a13d6a79b633c5b82/src/Avalonia.Themes.Fluent/Controls/Expander.xaml#L90

I think it should have ColumnDefinitions="Auto,*" . Should I make a pull request with this fix?

[maxkatz6]

Btw. It’s a problem with default expander style. For fluent you can avoid it without changing whole template:

  <Style Selector="Expander /template/ ToggleButton#ExpanderHeader">
    <Setter Property="HorizontalContentAlignment" Value="Stretch" />
  </Style>

But for convenience I will change default value anyway. It makes more sense.

[maxkatz6]

The best option would be to adapt Expander style from WinUI repo but only for Fluent theme. For now though your solution with ColumnDefinition looks good to me. PRs are welcome.