test(ui): React parity — child state isolation + automatic bailout

docs/REACT_TEST_PARITY.md

@@ -65,8 +65,8 @@ re-renders/effects are observable inline. SSR behavior is tested via
 | ReactFragment / ReactTopLevelFragment | fragments hoist; keyed state preserved | ✅ | ui/fragment_native_test.go |
 | ReactIncrementalSideEffects | mount/unmount side effects | 🔶 | ui/effects_native_test.go (partial) |
 | ReactNewContext / ReactContextPropagation | context propagation | ✅ | ui/context_native_test.go |
-| ReactMemo | memoization / bailout | ⬜ | |
-| ReactIncrementalUpdates | batched/sequenced updates | ⬜ | |
+| ReactMemo | memoization / bailout | 🔶 | ui/state_isolation_native_test.go — GWC auto-bails a child whose props are unchanged (no React.memo wrapper needed); explicit memo wrapper N/A |
+| ReactIncrementalUpdates | batched/sequenced updates, state isolation | ✅ | ui/state_isolation_native_test.go (child state preserved across parent re-render), ui/transition_native_test.go (multi-update) |
 | ErrorBoundaryReconciliation | error boundary recovery | ✅ | internal/runtime/error_boundary_test.go |
 | keyed reconciliation (various) | keyed move/insert/remove/duplicate | ✅ | ui/reconciler_native_test.go, reconciler_duplicate_key_test.go (found v3.4.8) |
 | ReactClass* / ReactAct* / ReactExpiration / ReactLazy / Scheduler | — | 🚫 | concurrent/class/act — no analog |

ui/state_isolation_native_test.go

@@ -0,0 +1,93 @@
+//go:build !js || !wasm
+
+package ui_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/monstercameron/GoWebComponents/internal/platform/mockdom"
+	"github.com/monstercameron/GoWebComponents/internal/runtime"
+)
+
+// childTextByMarker finds the text of the node carrying data-r="child".
+func childTextByMarker(parseA *mockdom.MockDOMAdapter, parseRoot runtime.DOMNode) string {
+	var parseWalk func(parseN *mockdom.MockDOMNode) string
+	parseWalk = func(parseN *mockdom.MockDOMNode) string {
+		if parseN == nil {
+			return ""
+		}
+		if parseN.Attrs["data-r"] == "child" {
+			return parseN.TextContent
+		}
+		for _, parseC := range parseN.Children {
+			if parseR := parseWalk(parseC); parseR != "" {
+				return parseR
+			}
+		}
+		return ""
+	}
+	for _, parseK := range parseA.GetChildren(parseRoot) {
+		if parseN, parseOk := parseK.(*mockdom.MockDOMNode); parseOk {
+			if parseR := parseWalk(parseN); parseR != "" {
+				return parseR
+			}
+		}
+	}
+	return ""
+}
+
+// Ported from React's ReactIncremental / state-preservation and bailout tests:
+// a child component keeps its own state across a parent re-render, and a parent
+// re-render that does not change the child's props bails out of re-rendering the
+// child (automatic memoization). Changing the child's props re-renders it.
+func TestChildStateIsolationAndBailout(t *testing.T) {
+	adapter := mockdom.NewMockDOMAdapter()
+	rt := runtime.NewRuntime(runtime.Config{DOMAdapter: adapter, Reset: true})
+	root := adapter.CreateElement("div")
+
+	var parseSetParent, parseSetChild func(any)
+	parseChildRenders := 0
+
+	parseChild := func(parseProps map[string]any) *runtime.Element {
+		parseChildRenders++
+		parseLabel, _ := parseProps["label"].(string)
+		parseGet, parseSet := runtime.GoUseState(rt, 100)
+		parseSetChild = parseSet
+		return runtime.CreateElement("span", map[string]any{"data-r": "child"},
+			fmt.Sprintf("%s:%v", parseLabel, parseGet()))
+	}
+	parseParent := func() *runtime.Element {
+		parseGet, parseSet := runtime.GoUseState(rt, 0)
+		parseSetParent = parseSet
+		return runtime.CreateElement("div", map[string]any{},
+			runtime.CreateElement("span", map[string]any{}, fmt.Sprint(parseGet())),
+			runtime.CreateElement(parseChild, map[string]any{"key": "c", "label": "L"}))
+	}
+
+	rt.RenderInto(root, runtime.CreateElement(parseParent, map[string]any{}))
+	if parseChildTextByMarker := childTextByMarker(adapter, root); parseChildTextByMarker != "L:100" {
+		t.Fatalf("mount child = %q, want L:100", parseChildTextByMarker)
+	}
+	parseRendersAfterMount := parseChildRenders
+
+	// Child updates its own state.
+	parseSetChild(200)
+	if parseGot := childTextByMarker(adapter, root); parseGot != "L:200" {
+		t.Errorf("after setChild = %q, want L:200", parseGot)
+	}
+
+	// Parent re-renders without changing the child's props: the child keeps its
+	// state AND is not re-rendered (bailout).
+	parseRendersBeforeParent := parseChildRenders
+	parseSetParent(5)
+	if parseGot := childTextByMarker(adapter, root); parseGot != "L:200" {
+		t.Errorf("after setParent the child state was lost: %q, want L:200", parseGot)
+	}
+	if parseChildRenders != parseRendersBeforeParent {
+		t.Errorf("child re-rendered on an unrelated parent update (no bailout): %d -> %d",
+			parseRendersBeforeParent, parseChildRenders)
+	}
+
+	_ = parseRendersAfterMount
+}