"Menu" control doesn't work when the Modern Theme is selected

[giovanni-a]

To reproduce:

Use the following code when the Modern Theme is selected. The "File" menu appears but it cannot be opened.

    <Grid>
        <Menu VerticalAlignment="Top">
            <MenuItem Header="File">
                <MenuItem Header="New" />
                <MenuItem Header="Open" />
                <MenuItem Header="Save" />
                <Separator />
                <MenuItem Header="Exit" />
            </MenuItem>
        </Menu>
    </Grid>

Links to reproduce:

• Version that works fine (Silverlight Theme): https://xaml.io/s/giovanni_a/MenuItemIssue_SLTheme

• Version that doesn't work (Modern Theme): https://xaml.io/s/giovanni_a/MenuItemIssue_ModernTheme