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The problem is related with TitleBar. you set ExtendViewIntoTitleBar as true, then the navigationview will fill the full window, but you have not specific the custom title bar, so the system 32 height titlebar will hover the Toggle button.
For this scenario, please refer to Title bar customization doc. set the custom titlebar margin left as 50 to make sure it will not hover navigationview toggle button.
<Grid
x:Name="AppTitleBar"
Margin="50,0,0,0"
VerticalAlignment="Top"
Background="Transparent">
<Grid.ColumnDefinitions>
<ColumnDefinition x:Name="LeftPaddingColumn" Width="0" />
<ColumnDefinition />
<ColumnDefinition x:Name="RightPaddingColumn" Width="0" />
</Grid.ColumnDefinitions>
<Image
Grid.Column="1"
Width="20"
Height="20"
Margin="12,0"
HorizontalAlignment="Left"
Source="Assets/Square44x44Logo.png" />
<TextBlock
Grid.Column="1"
Margin="50,8,0,0"
Style="{StaticResource CaptionTextBlockStyle}"
Text="Custom Title Bar" />
</Grid>
Please note, you need to set the title bar z-index top of current window.
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