db-mainnavigation is completely broken below 1023 px viewport

[mriehl]

When viewing the db-mainnavigation component in the storybook and resizing the window to be less than 1023px width the menu will completely disappear.
I tested this in Chrome and Safari, not sure about Firefox.

There are various display: none styles attached to it conditionally.

Patching the core css with

@media screen and (max-width: 1023px) {
  .cmp-mainnavigation > ul {
    display: block !important;
  }
}

at least makes it "somewhat" useable but obviously patching the CSS is an awful solution I don't want to rely on.
Could you please check if the current css behaviour is really correct.

[mriehl]

@mfranzke the merge request storybook looks good. Is there any way to get this into the main branch and release the lib?