fix(helpers): guard addChild against re-inserting an already-linked node

[terminalchai]

Problem

Closes #1138

When overlapping replace tweens are created on the same target/property in rapid succession (e.g. promise callbacks racing with document.hidden foreground events), addChild can be called with a tween that is already present in the composition replace list. This produces a self-referential linked list cycle (tween._prevRep === tween) that freezes the animation engine update loop, causing a tab hang.

The issue was observed in devtools as the same tween ID repeating thousands of times in the _prevRep chain.

Root cause

addChild inserts unconditionally. If child is already in the list its _prevRep/_nextRep pointers are non-null, so the sorted-walk while (prev && sortMethod && ...) will eventually reach the child itself as prev. At that point:

prev[nextProp] = child  // child._nextRep = child  ← cycle!

Fix

Add an early guard at the top of addChild that detects whether the child is already linked and calls removeChild first:

if (parent._head === child || child[prevProp] != null) {
  removeChild(parent, child, prevProp, nextProp);
}

• parent._head === child — child is the head of the list (its _prevRep is null but it is already inserted)
• child[prevProp] != null — child has a predecessor, meaning it is mid-list or tail

removeChild cleanly unlinks the node (setting both pointers to null) before addChild re-inserts it at the correct sorted position. When called with the default _prev/_next props this also protects the main engine ticking list against the same edge case.

Files changed

• src/core/helpers.js — source fix
• dist/ — rebuilt