GroupBox may have mousewheel event response problems when its contents include a scrollviewer

Hello,
GroupBox has its own extended scrolling mechanism (it doesn't contain its own ScrollViewer) that also handles the mouse wheel action. So, although this behavior can be undesired, it is expected. Also, GroupBox is designed to be used only inside LayoutControl. So, if you use this element as a separate container for DockLayoutManager, I recommend that you replace it with LayoutControl and disable its scrolling logic by setting the attached ScrollBarExtensions.AllowMouseScrolling property to false:

XAML
<dxlc:LayoutControl  Grid.Row="0" Grid.Column="0"  dx:ScrollBarExtensions.AllowMouseScrolling="False">
    <dxdo:DockLayoutManager x:Name="DockManager1"
                        FloatingMode="Desktop" AllowCustomization="False"
                        AllowMergingAutoHidePanels="False" ManualClosedPanelsBarVisibility="True"
                        ClosedPanelsBarPosition="Bottom" ClosedPanelsBarVisibility="Auto">
        <dxdo:LayoutGroup>
            <dxdo:DocumentGroup>
                <dxdo:DocumentPanel  Caption="TestScrollView">
                    <Grid Margin="0,0,0,0">
                        <local:ImageViewer x:Name="Viewer1" />
                    </Grid>
                </dxdo:DocumentPanel>
            </dxdo:DocumentGroup>
        </dxdo:LayoutGroup>
    </dxdo:DockLayoutManager>
</dxlc:LayoutControl>

If you still need to use the GroupBox element, put it to LayoutControl and set the ScrollBarExtensions.HandlesDefaultMouseScrolling property to "False" inside your UserControl:

XAML
<ScrollViewer x:Name="ScrollBar"
                dx:ScrollBarExtensions.HandlesDefaultMouseScrolling="False"
                HorizontalScrollBarVisibility="Visible"
                VerticalScrollBarVisibility="Visible">
    <Grid>
        <Image x:Name="ImageCaptured" HorizontalAlignment="Center" VerticalAlignment="Center"
                RenderOptions.BitmapScalingMode="NearestNeighbor"
                RenderOptions.ClearTypeHint="Enabled"
                UseLayoutRounding="True"
                SnapsToDevicePixels="True" />
    </Grid>
</ScrollViewer>

Thanks,
Kirill