Layout issue with TabControl w/ TabStripPlacement Left, TabControlHelper.IsUnderlined="True"

[aplocher]

What steps will reproduce this issue?

<controls:MetroTabControl TabStripPlacement="Left" DockPanel.Dock="Top" x:Name="tabControl1" controls:TabControlHelper.IsUnderlined="True">
<TabItem>
    <TabItem.HeaderTemplate>
        <DataTemplate>
            <TextBlock Background="#eeeeee">Some longer text</TextBlock>
        </DataTemplate>
    </TabItem.HeaderTemplate>
</TabItem>
<TabItem Header="Short"></TabItem>
<TabItem Header="Short 2"></TabItem>
<TabItem Header="Mister Longie McLongerson VII">
    <StackPanel>
        <TextBlock>Hi</TextBlock>
    </StackPanel>
</TabItem>
</controls:MetroTabControl>

Expected outcome

I believe the "underlines" (or in this case, side-lines) should line up when you have a TabControl with IsUnderlined = True and the TabStripPlacement=Left. I actually really like the idea of the lines being off to the side like this, but they don't look right when they're not aligned.

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Environment

• MahApps.Metro __(not sure off top of my head, but latest in nuget as of today)
• Windows 10
• Visual Studio 2017 RC1
• .NET Framework 4.5.2