I can't scroll to my scrollviewer, it disappear when I move the mouse. Was it because it was just a tooltip? If yes, is there a way to scroll the text in my tooltip?

<TextBlock TextWrapping="Wrap" Style="{StaticResource TextBlockWidthStyle}" 
                                   Text="{Binding ExtendedDescription}"
                                   TextTrimming="WordEllipsis">
                                <ToolTipService.ToolTip>
                                    <ScrollViewer Width="310" VerticalScrollBarVisibility="Auto">
                                        <TextBlock Text="{Binding ExtendedDescription}"
                                                   Width="300"
                                                   TextWrapping="Wrap"/>    
                                    </ScrollViewer>
                                </ToolTipService.ToolTip>
                            </TextBlock>