area layout covers affixed navigation in right sidebar - links in chrome are not selectable

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i have a modified "elemental"-theme, running on concrete5, version 7.5.6
Theme Provider Interface is correctly set.
i make use of layout presets:

array(
                'handle' => 'mg_7-3',
                'name' => 'mg_7-3',
                'container' => '<div class="row"></div>',
                'columns' => array(
                    '<div class="col-sm-8 col-md-7 col-xs-12"</div>',
                    '<div class="col-sm-4 col-md-3 col-xs-12"</div>'
                ),
            ),
            array(
                'handle' => 'mg_10',
                'name' => 'mg_10',
                'container' => '<div class="row"></div>',
                'columns' => array(
                    '<div class="col-sm-12 col-md-10"</div>',


my pagetemplate looks like this:
<main>
    <div class="container">
        <div class="row">
            <div class="col-sm-10 col-content">
                <?php
                $a = new Area('Main');
                $a->setAreaGridMaximumColumns(12);
                $a->display($c);
                ?>
            </div>
        </div>
    </div>
</main>


in the header i have to the right a logo and in the sidebar to the right a vertical main navigation. i use the affix rule to fix the navigation on the top position.


now, in crome or older safari-versions the underlying area layout covers the navigation. the pages in the navigation are not selectable.
i tried to use a nested layout in my first column, but then the column-alignement is gone. i also tried to assign the z-axis to 999 to block, area etc. without success.
firefox and newer safari version are not affected.

- is there a workaround for that? or is it even a bug in chrome?

url is:http://www.marcoganz.ch

thanks in advance for any help
blink.

blinkdesign