[FIX] Fix nested array isolation in Observer constructor

[willeastcott]

Fixes playcanvas/editor#684

Overview

The Observer class was not properly isolating nested arrays from source data, causing issues when the source data was modified externally (e.g., by ShareDB during real-time collaboration).

The Problem

When creating an Observer from data containing nested arrays (like [[1,2,3], [4,5,6]]), the inner arrays were not being copied - only the outer array was shallow copied. This meant the Observer and the source data shared references to the same inner arrays.

In the Editor, this caused script attributes with array types (e.g., vec3[]) to fail live-updating in the Launch tab:

1. User changes a vec3 array attribute in the Inspector
2. ShareDB receives the operation and updates its document data
3. Because the Observer shared the same inner array reference, its data was also modified
4. When observer.set() was called, the value was already the same, so no *:set event fired
5. The Launch tab never received the update

The Fix

• Added deepCopyArray() utility function in utils.ts (alongside existing arrayEquals)

• Fixed line 205 in _prepare() which was calling .slice(0) but discarding the result:
  // Before (bug)
  target._data[key][i].slice(0);

  // After (fixed)
  target._data[key][i] = deepCopyArray(target._data[key][i]);

3 unit tests have been added. All fail without the fix but now pass.

The original Editor issue is confirmed fixed.

[Copilot]

Pull request overview

This pull request fixes a bug where nested arrays were not properly isolated in the Observer constructor, causing issues with real-time collaboration via ShareDB. When source data was modified externally, the Observer's internal data was inadvertently modified as well, preventing change events from firing correctly.

Changes:

• Added deepCopyArray() utility function to recursively copy nested arrays
• Fixed line 205 in _prepare() method to use deepCopyArray() instead of the buggy .slice(0) call
• Added comprehensive test coverage for nested array isolation scenarios

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/utils.ts	Added deepCopyArray() utility function with proper documentation for deep copying nested arrays
src/observer.ts	Fixed nested array isolation bug in _prepare() method by using deepCopyArray() instead of discarding .slice(0) result
test/observer.test.mjs	Added comprehensive test suite covering nested array isolation, including the exact bug scenario from issue #684

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