Fixed: Submenus don't show scroll viewer if items exceed the available space on screen

[floele-ww]

Test case:

<TextBox>
                    <TextBox.ContextMenu>
                        <ContextMenu>
                            <MenuItem>
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            <MenuItem Header="A" />
                            </MenuItem>
                        </ContextMenu>
                    </TextBox.ContextMenu>
                </TextBox>

[batzen]

I guess this should better be fixed in the MenuScrollViewer style.
That way it get's fixed in all themes and controls at once.
Fixed it there now.

Thanks for contributing!

[floele-ww]

Sounds reasonable, thanks!