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Ok, Thanks to Dragan and many many tests I actually did figure it out. This is where the problem was:

            <ScrollViewer
                Grid.Column="2"
                HorizontalScrollBarVisibility="Auto"
                VerticalScrollBarVisibility="Auto">
                <Frame
                    x:Name="_mainFrame"
                    Grid.Column="2"
                    Margin="0"
                    HorizontalAlignment="Stretch"
                    VerticalAlignment="Stretch"
                    NavigationUIVisibility="Visible" />                    
            </ScrollViewer>

The ScrollViewer broke the widths... As soon as I removed that, all works as expected.

Dragan, I would like you to get the credit for this solution - how do I do that?

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Posted by: Danie Spreeth

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