Here's a Playground where you can observe the leak:

Playground

Click the button a few times and then take a Heap Snapshot. Filtering on VClass, you should see something like this:

The instance with distance 7 is supposed to be there. That is a current dependency of the computed property. But the other instances, with distance 10, shouldn't be there. They are no longer in use and should have been garbage collected.

The targetMap shown in the Retainers is from effect.ts. That's a WeakMap. The keys in that map are reactive objects, so in my example it would be the thing I've called set. If the set object is GCed then the leak will be cleaned up, but in cases where that object is retained long term, e.g. within a global store, the WeakMap won't save us.

The value inside the WeakMap is a Map. The key for this Map is the property being tracked. The corresponding value is a Set of effects that are tracking that property, referred to as Dep in the types.

When a dependency is removed from an effect, it is removed from that Set of effects. However, if that Set is now empty, it isn't removed. That is a small leak, but arguably not worth the effort to tidy up.

The potential for a much bigger leak occurs when the reactive object is a Set or a Map. In that case, the 'property' being tracked can also be an object. So while the empty Set of dependencies may only be a small leak, the leak of the key can be an arbitrary, and potentially large, object. In my example it is the relatively small VClass object, but in principle that object could be much bigger.

To put it another way:

const targetMap = new WeakMap<any, Map<any, Set<ReactiveEffect>>>()
//                                     ^^^
//                                      |
//                         This 'any' can be an object
//                         that leaks when the
//                         Set<ReactiveEffect> is empty

The leaking objects don't need to have ever been in the reactive Set or Map. It's enough just to call has(), as that establishes a dependency on them, even if they are never in the collection.

Here is the same example but using the code in this PR:

Playground - leak fixed