Problem

Effects created inside $derived statements are not re-executed when the derived is accessed after being disconnected and reconnected to the reactive dependency graph.

Reproduction scenario:

1. Component A creates a derived with effects: const result = $derived(/* contains $effect */)
2. Component A unmounts before effects have a chance to execute
3. Component B tries to access the same derived
4. The effects are never executed, even though the derived value is computed correctly

This happens because when a derived is disconnected from the graph (no more reactions), its effects are destroyed via destroy_derived_effects(). When the derived is later reconnected, the effects are not recreated since the derived appears "clean" and doesn't need re-execution.

works here: https://svelte.dev/playground/719a2a834fd34d02a204ac4b7c65973b?version=5.19.4
does not work: https://svelte.dev/playground/719a2a834fd34d02a204ac4b7c65973b?version=5.19.5

Solution

Introduce a HAS_EFFECTS flag that tracks when a derived has contained effects, even after they've been destroyed. During reconnection, if a derived has the HAS_EFFECTS flag but no current effects, we force re-execution to recreate them.

Changes:

1. Add HAS_EFFECTS = 1 << 24 constant
2. Set the flag in effects.js when effects are added to deriveds
3. Check for missing effects in runtime.js and trigger re-execution when needed

Implementation

// In effects.js - mark derived as having effects
var derived = /** @type {Derived} */ (active_reaction);
(derived.effects ??= []).push(effect);
derived.f |= HAS_EFFECTS;

// In runtime.js - detect and fix missing effects
if (is_dirty(derived)) {
    update_derived(derived);
} else if ((derived.f & HAS_EFFECTS) !== 0 && (derived.effects === null || derived.effects.length === 0)) {
    // If the derived once had effects but they're now missing (destroyed),
    // we need to re-execute to recreate them
    update_derived(derived);
}

Testing

Added a comprehensive test that:

• Creates a derived with effects in one render context
• Destroys the context before effects execute (simulating component unmount)
• Accesses the derived from a new render context
• Verifies effects execute properly on reconnection

Without fix: 0 effect executions (effects lost forever)
With fix: 1 effect execution (effects recreated on reconnection)

Impact

• ✅ Fixes the reported bug with zero performance impact for deriveds without effects
• ✅ Maintains backward compatibility
• ✅ All existing tests continue to pass
• ✅ Minimal code change with clear intent