test(ui): React parity — StartTransition applies updates

docs/REACT_TEST_PARITY.md

@@ -50,7 +50,7 @@ re-renders/effects are observable inline. SSR behavior is tested via
 | SSR primitive children (number/bool/null) | render primitives as children | 🚫 | Go static typing: html.* children are ui.Node; primitives go via ui.Text |
 | onlyChild-test | Children.only | ⬜ | |
 | ReactContextValidator / NewContext | context | ✅ | ui/context_native_test.go |
-| ReactStartTransition | transitions | ⬜ | (GWC has StartTransition) |
+| ReactStartTransition | transitions | ✅ | ui/transition_native_test.go (transition-wrapped updates commit; multi-update; same-value dedup) |
 | ReactJSX* / ES6Class / PureComponent / Version / Profiler-devtools | — | 🚫 | no Go analog |
 
 ## react-reconciler (spirit-applicable)

ui/transition_native_test.go

@@ -0,0 +1,60 @@
+//go:build !js || !wasm
+
+package ui_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/monstercameron/GoWebComponents/internal/platform/mockdom"
+	"github.com/monstercameron/GoWebComponents/internal/runtime"
+)
+
+// Ported from React's ReactStartTransition: a state update wrapped in
+// StartTransition is applied (it commits to the host DOM); a same-value
+// transition update does not re-render; multiple updates inside one transition
+// all take effect.
+func TestStartTransitionAppliesUpdates(t *testing.T) {
+	adapter := mockdom.NewMockDOMAdapter()
+	rt := runtime.NewRuntime(runtime.Config{DOMAdapter: adapter, Reset: true})
+	root := adapter.CreateElement("div")
+
+	var parseSetA, parseSetB func(any)
+	rt.RenderInto(root, runtime.CreateElement(func() *runtime.Element {
+		parseGetA, parseSA := runtime.GoUseState(rt, 0)
+		parseGetB, parseSB := runtime.GoUseState(rt, 0)
+		parseSetA = parseSA
+		parseSetB = parseSB
+		return runtime.CreateElement("span", map[string]any{}, fmt.Sprintf("%v-%v", parseGetA(), parseGetB()))
+	}, map[string]any{}))
+
+	read := func() string {
+		parseNode, _ := adapter.GetChildren(root)[0].(*mockdom.MockDOMNode)
+		return parseNode.TextContent
+	}
+
+	if read() != "0-0" {
+		t.Fatalf("mount = %q, want 0-0", read())
+	}
+
+	// A single transition update commits.
+	rt.StartTransition(func() { parseSetA(7) })
+	if read() != "7-0" {
+		t.Errorf("after StartTransition setA(7) = %q, want 7-0", read())
+	}
+
+	// Multiple updates inside one transition all take effect.
+	rt.StartTransition(func() {
+		parseSetA(1)
+		parseSetB(2)
+	})
+	if read() != "1-2" {
+		t.Errorf("after multi-update transition = %q, want 1-2", read())
+	}
+
+	// A same-value transition update is a no-op (final value unchanged).
+	rt.StartTransition(func() { parseSetA(1) })
+	if read() != "1-2" {
+		t.Errorf("after same-value transition = %q, want 1-2", read())
+	}
+}