Allow collapsing overflow area of ToolBar

Is your feature request related to a problem? Please describe.
A ToolBar always contains the area for the overflow, even there is no overflow at all. I am not sure if this is intended behavior or no. For small toolbars I find this rather annoying.

Here a small example, the overflow part is the lighter area to the right of the ‘B’ button:

In the package code itself I can get achieve my expected result by changing ToolBarOverflowButtonVisibilityConverter to return Visibility.Collapsed instead of Visibility.Hidden. But I guess that could cause issues in other use cases.

Describe the solution you'd like
The behavior of the overflow area is configurable in XAML.

Describe alternatives you've considered
My current workaround is to collapse the overflow grid using an implicit style:

<ToolBar.Resources>
    <Style TargetType="{x:Type Grid}">
        <Setter Property="Visibility" Value="Collapsed" />
    </Style>
</ToolBar.Resources>

This works only as long as there are no other uses of Grid.

Additional context
This is based on to 4.5.0.

まず、issue で参照されている ToolBar テンプレートと ToolBarOverflowButtonVisibilityConverter を見つけ、XAML でオーバーフロー領域がどのように表示されるかを確認します。他の Grid の使用箇所を変更せずにその領域を折りたたむための、設定可能な XAML behavior を追加し、オーバーフローのあるツールバーとないツールバーが期待どおりにレンダリングされることを確認します。