Setting scrollTop does not work while momentum scroll is ongoing. But there is a workaround.

Environment iOS Safari

Phenomenon

• User gesture
  • Do momentum(aka inertial) scrolling
• code
  • Fire onScroll event as long as inertial momentum scrolling

Problem

• Setting scrollTop does not work while momentum scroll is ongoing
  • Manually setting scrollTop is impossible while momentum scrolling
  • So events that have dependencies on scrollTop value and manipulate scrollTop at the same time could work incorrectly

Solution

• solution: setTimeout(onScrollEvent, 0)
  • There is not any perfect solution for the problem
  • But workaround is here. why?
    • While momentum scroll is ongoing,
    • Before applying the solution
      • ScrollTop value is changed in real time while momentum scrolling is ongoing
      • The changed scrollTop value make onScrollEvent be fired as soon as possible
    • After applying the solution
      • The onScrollEvent will be fired after other tasks like rendering

20210809