When a RadSplitContainer with Orientation="Vertical" contains RadPaneGroup instances, resizing them causes a flicker when the ShowResizePreview property is set to False. This happens with the VisualStudio2019 theme.
For the time being, a possible workaround is to create a global Style with TargetType="RadPaneGroup" and set the Margin property to "0":
<Style TargetType="telerik:RadPaneGroup" BasedOn="{StaticResource RadPaneGroupStyle}">
<Setter Property="Margin" Value="0"/>
</Style>