In  issue 663291  it was discovered that when we have a mix of composited fixed position and non-composited fixed position elements they can be offset by a pixel during scroll. The cause is that we sync the truncated scroll offset to blink but round the snapped scroll offset in cc which results in composited fixed position objects being fixed to a different offset than non-composited fixed position objects.

When the scroll offset was rounded to attempt to be consistent on low DPI, it introduced  issue 677686  where we had unstable scroll positions.

We need to ensure the synced scroll offset matches the snapped offset used for composited fixed position elements so that we can properly render a mix of composited and non-composited fixed position elements.

To see this bug on tip of tree, go to http://www.bcci.tv/ and add will-change: transform; to the side fixed element with class .liveBar: i.e. document.querySelector('.liveBar').style.willChange = 'transform'; This will force the side bar to be composited while the top one is updated on main and after scrolling up/down a white line will appear between the two as in  issue 663291 .