⚡ Quick Fix (TL;DR)

The Culprit: An element somewhere on the page is rendering wider than the actual viewport.

The Fix: Hide horizontal overflow at the body level. Use this carefully as a general habit, since it can conceal a real underlying layout problem rather than fixing it – but on this site specifically, it is also the confirmed, deliberate fix for the known full-bleed 100vw scrollbar issue that several pages have hit (see the matching Error Guide entry): when a full-bleed element is the actual cause, this is the correct fix, not a workaround.

body {
  overflow-x: hidden;
}