When the anchor row is gone and a virtual top-spacer is present but the snapshot
carries no captured topPadBefore (null), Number(null) is 0, so the topPad-delta
branch would add the ENTIRE current spacer height to scrollTop and fling the reader
far from their content. Guard on an actual (non-null) captured topPadBefore; else
keep the raw fallback target. Mutation-checked (drop the null guard -> the reader is
flung 1300px and the test fails).
allenliang2022
fbfb6da8fe
