Created attachment 450368 [details]
Miminal test case
Having an element negatively positioned outside the viewport, negative contextual positioning (i.e. the immediate ancestor is relatively positioned)
disables vertical scrolling on the whole document when the content of the positioned element has been hidden via the overflow property.
Minimal test case (attached):
Looks like we hit LayoutUnit limits when computing overflow.
Simple workaround is to use smaller negative offset.