fix(mobx): ObservableSet union/intersection/symmetricDifference result ordering

[spokodev]

ObservableSet implements Set<T>, so its ES2024 set methods should match native Set result ordering: union appends the argument's new elements after the receiver's, and intersection and symmetricDifference keep the receiver's order. When the argument is a plain (non-observable) Set, union, intersection and symmetricDifference instead delegate to the argument (otherSet.method(this)), so the result comes out in the argument's order.

const s = observable.set([1, 2, 3])
;[...s.union(new Set([3, 4]))]        // was [3, 4, 1, 2], native is [1, 2, 3, 4]
;[...s.intersection(new Set([3, 2]))] // argument-ordered instead of receiver-ordered

Values and membership are correct, so this slipped past the existing tests, which compare with toEqual(new Set([...])) (order-insensitive). It surfaces whenever the result is iterated (spread, Array.from, forEach, rendering a list) after moving from a plain Set to observable.set.

Fix

Build the result from the receiver in every case: return new Set(this).method(otherSet), matching the already-correct difference, isSubsetOf and isSupersetOf. The observable-argument case already used this form (the previous else branch), so the #3919 stack-overflow behavior is unchanged; only the plain-Set path changes, from argument-order to receiver-order. isDisjointFrom is aligned to the same form as well (its boolean result was already correct, so this is only for consistency).

Testing

Added an order-sensitive test to __tests__/base/set.js that compares against a native Set built from the same receiver. It fails before this change; the full set suite (28 tests, including the #3919 case) stays green with it.

[changeset-bot]

🦋 Changeset detected

Latest commit: cf9f892

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
mobx	Patch

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[Copilot]

Pull request overview

This pull request fixes ObservableSet’s ES2024 Set-method result ordering (union, intersection, symmetricDifference) so it matches native Set semantics when the argument is a plain (non-observable) Set, and aligns isDisjointFrom to the same delegation pattern for consistency.

Changes:

• Update ObservableSet ES2024 set methods to always compute results from the receiver via new Set(this).<method>(otherSet), ensuring receiver-based ordering.
• Add an order-sensitive test that compares ObservableSet iteration order against a native Set copy using the same operations.
• Add a patch changeset documenting the ordering fix.

Reviewed changes

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

File	Description
packages/mobx/src/types/observableset.ts	Ensure ES2024 Set operations on ObservableSet preserve receiver-based result ordering by delegating through new Set(this) consistently.
packages/mobx/tests/base/set.js	Add an order-sensitive regression test to verify behavior matches native Set when operating with a plain Set argument.
.changeset/fix-observableset-receiver-order.md	Document the behavioral fix as a patch-level change.

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