Why it matters
Shifts that happen more than 500 ms after a tap are not excused by user input and feel like the page is jumping on its own.
How to fix it
Reserve space for content revealed by interactions, and animate reveals with transform/opacity or grid-template-rows 0fr→1fr.
More smoothness rules
void smooth reports 13 rules in this category. How the page feels after it loads: dropped frames during a scripted scroll, long animation frames, INP of real clicks, shifts after input, animated layout properties, idle rAF loops and reduced-motion handling.
scroll-jankFrames are dropped while scrollinglong-frames-during-scrollLong animation frames block scrollingscroll-listener-nonpassiveNon-passive wheel/touch listenerinp-slowInteractions are slow to respond (INP)layout-shift-on-scrollLayout shifts while scrollinganimate-layout-propertyAnimation of a layout-triggering propertytransition-alltransition: allwill-change-overusewill-change is applied too broadly