How to render a thin GridSplitter?
By default, MinWidth of the WPF Grid Splitter is 25, if we set width below than that, control takes the MinWidth to arrange the GridSplitter. To render a thin GridSplitter, we must change the MinWidth of the SfGridSplitter to the required size.
Mainwindow.xaml:
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition />
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Calendar HorizontalAlignment="Center" VerticalAlignment="Center" />
<syncfusion:SfGridSplitter x:Name="gridsplitter" Grid.Column="1" Width="5" MinWidth="5"/>
<Slider Grid.Column="2" HorizontalAlignment="Stretch" Maximum="100" ToolTip="{Binding RelativeSource={RelativeSource Self}, Path=Value}" VerticalAlignment="Center" Value="50" />
</Grid>
Screenshot:

Sample:
https://www.syncfusion.com/downloads/support/directtrac/general/ze/GridSplitter_Sample1818918199.zip