Under certain circumstances the collapsing top navigation bar will cause the screen to flash very quickly when scrolling. I am unable so far to create a video of this phenomenon, but I have attached two screenshots to attempt to show what is happening.
I believe this issue appears when the pixel height of the content that is outside the scroll-able area of the page is roughly the same as the reduction in height caused by the collapsing/shrinking of the top bars. When the top bars shrink the height of the page is reduced. If the new total height of the page is smaller than the height of the browser window the scroll bar will disappear, but this also makes the top bars re-inflate. This happens 3-4 rapidly causing bad screen flashing.