>Something like:
>

><ScrollViewer HorizontalScrollBarVisibilitty="Auto" VerticalScrollBarVisibility="Auto">
>    <TabControl>
>        <TabItem>
>            <Grid>
>                <Grid.ColumnDefinitions>
>                    <ColumnDefinition Width="300"/>
>                    <ColumnDefinition/>
>                </Grid.ColumnDefinitions>
>                <StackPanel>
>                        <!-- Main Content, allow horizontal scrolling, don´t allow vertical scrolling-->
>                </Stackpanel>
>                <StackPanel Grid.Column="1">  // Allow Vertical Scrolling
>                     <Expander>
>                            <!-- Description 1 -->
>                     </Expander>                     
>                     <Expander>
>                            <!-- Description 2 -->
>                     </Expander>                     
>                     <Expander>
>                            <!-- Description 3 -->
>                     </Expander>                     
>                </StackPanel>
>            </Grid>
>        </TabItem>
>        ...
>    </TabControl>
></ScrollViewer>
>

Look back at Viv's answer, that she wrote for you.

What I can't figure out is why you are put a TabControl inside of a ScrollViewer. If you do that
your tabs are going to scroll out of site which is generally not what is wanted in most designs.

You want the ScrollViewer to be inside the TabControl and in this
case you want two of them, one for the left side and one the right.

Again look at Viv's code what she did should work for what you are describing.