Overview

SplitView class (microsoft.com)
SplitView controls have an expandable / collapsible pane and a content area:

SplitView’s content area is always visible. The pane has four modes:

1. Overlay — hidden until opened; when opened, overlays content area
2. Inline — always visible, does not overlay content area
3. CompactOverlay — narrow portion (just wide enough to show icons) is always visible; when opened, overlays content area
4. CompactInline — same as CompactOverlay but, when opened, pushes content out of its way

<SplitView IsPaneOpen="True"
    DisplayMode="Inline"
    OpenPaneLength="296">
    <SplitView.Pane>
        <TextBlock Text="Pane"
        FontSize="24"
        VerticalAlignment="Center"
        HorizontalAlignment="Center"/>
    </SplitView.Pane>

    <Grid>
        <TextBlock Text="Content"
        FontSize="24"
        VerticalAlignment="Center"
        HorizontalAlignment="Center"/>
    </Grid>
</SplitView>