Fix memory leak in AnimatedProps constructor

[c-miles]

Fixes a memory leak in AnimatedProps that causes unbounded accumulation of child nodes in applications with frequent re-renders.

Fixes #2756. Builds on recent improvements in this area like #2800.

Background

During investigation of facebook/react-native#49217, I discovered this issue is specific to react-native-web. This area of the vendored Animated implementation has diverged from React Native core, where the AnimatedProps constructor does not call __attach().

Root Cause

The AnimatedProps constructor calls this.__attach() (line 35), then useAnimatedPropsLifecycle calls node.__attach() again (line 110 in useAnimatedProps.js). This causes the same AnimatedProps instance to add itself as a child to parent nodes twice.

Each re-render creates a new AnimatedProps with this double-attachment, but cleanup only removes one reference. The result is unbounded accumulation of duplicate children. In a stress test with 500 animated components re-rendering every 50ms, this produced 359,382 duplicate __addChild calls.

Live demo of the issue (note: Android/iOs works fine, but web version exhibits the leak)

Approach

This PR removes the constructor __attach() call entirely.

Why this is safe:

• useLayoutEffect in useAnimatedProps (line 110) synchronously calls node.__attach() before browser paint
• No code path requires immediate attachment during construction
• React Native core's implementation proves this pattern works correctly

Initially considered adding a defensive check in __addChild() to prevent duplicates:

if (this._children.includes(child)) return

While this prevented the leak, it masked the underlying issue rather than addressing the root cause.

Impact

• Breaking changes: None
• Performance: Eliminates memory leak in animated applications
• Compatibility: Aligns behavior with React Native core implementation

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit e534ed9:

Sandbox	Source
react-native-web-examples	Configuration

[necolas]

Thanks!