I'm sure many of you have come across a situation where the Oxygen Header element, with the "Enable Sticky Header" option enabled, overlaps any objects on the site (being constantly in the topmost layer), including the Modal and the Gallery lightbox. The fact is that the initially sticky header has a huge z-index value. See example:
This is how it looks in the source code:
It would be better if the Oxygen team set the default z-index of the sticky header to around 100, for example. However, this is not the case yet and you have to manually change this value each time:
Here is such a small and non-obvious trick and your problem is solved.