Well very easy to describe as it’s also happening in your original codepen.io template. I couldn’t find anything in your documentation regarding this behaviour, hence I assume it’s a bug.
I am using Chrome Version 58.0.3029.81 and when resizing the width of the browser window (until the point when the vertical scrollbar goes away) it will start flickering. I have used this codepen template to verify this behaviour: https://codepen.io/pen?template=JXVYzq
Please see this video, which demonstrates the issue: https://streamable.com/22hi0
Am I missing something?
Webkit browsers like Safari and Chrome subtract the scroll bar width from the visible page width when calculating width: 100%% or 100vw. More at DM Rutherford’s Scrolling and Page Width.
Try using
overflow-y: overlayon parent.