This makes it impossible to have scrolling content over fixed content.
The attached project is just one scenario: the fixed side panel is rendered above the scrolling content.
Thank you for the report.
It's a known issue, but we're not able to fix it at the moment.
This issue is taking place from .sketch format and looking the same at the Sketch.
We'll make it more adequate as fast as some waited changes will come to the format structure.