fix: don't set derived values during time travel

packages/svelte/src/internal/client/reactivity/deriveds.js

@@ -353,9 +353,15 @@ export function update_derived(derived) {
 	var value = execute_derived(derived);
 
 	if (!derived.equals(value)) {
-		// TODO can we avoid setting `derived.v` when `batch_values !== null`,
+		// if we have batch_values we don't actually set the value of the derived
-		// without causing the value to be stale later?
+		// otherwise blocks that depends on that derived will not be considered dirty
-		derived.v = value;
+		// if the derived changes in a fork AND in a subsequent fork/normal state update
+		// IF we are in a tracking context the derived value of the derived will be set
+		// in the batch_values map thus updating it for this batch otherwise it will just be updated
+		// again during the `get` call
+		if (batch_values === null) {
+			derived.v = value;
+		}
 		derived.wv = increment_write_version();
 	}
 
@@ -371,7 +377,7 @@ export function update_derived(derived) {
 		// only cache the value if we're in a tracking context, otherwise we won't
 		// clear the cache in `mark_reactions` when dependencies are updated
 		if (effect_tracking()) {
-			batch_values.set(derived, derived.v);
+			batch_values.set(derived, value);
 		}
 	} else {
 		var status = (derived.f & CONNECTED) === 0 ? MAYBE_DIRTY : CLEAN;

packages/svelte/src/internal/client/runtime.js

@@ -614,6 +614,7 @@ export function get(signal) {
 	} else if (is_derived) {
 		derived = /** @type {Derived} */ (signal);
 
+		// if we already have a batched value don't bother updating it
 		if (batch_values?.has(derived)) {
 			return batch_values.get(derived);
 		}
@@ -625,6 +626,11 @@ export function get(signal) {
 		if (is_updating_effect && effect_tracking() && (derived.f & CONNECTED) === 0) {
 			reconnect(derived);
 		}
+
+		// we need to check again because we could've just update `batch_values` inside `update_derived`
+		if (batch_values?.has(derived)) {
+			return batch_values.get(derived);
+		}
 	} else if (batch_values?.has(signal)) {
 		return batch_values.get(signal);
 	}