Meaning I do have "Create Pages for Categories" enabled.
when you are viewing a specific category-page, then you click onto the small icon for "open quick navigation".
then it shows (for about half a second) only the blank background-color of my website, before the "quick navigation menu" is opening up.
Not sure whether is is some delay until the white-transparent overlay and the quick navigation opens. But it does not look great when seeing just the background-color for half a second......
This strange effect does not happen when clicking on the "quick navigation menu" at the homepage (Forum List). It only happens when viewing the categories (category-pages).