Snapshots render in a fresh child process: pixels no longer depend on test order

AGENTS.md

@@ -54,7 +54,7 @@ testdata/snapshots/  <os>/<name>-<light|dark>.png
 - `ui.NewTester` drives click, keys, typing. Assert values, not existence.
 - Every component: one `snapshot(t, name, w, h, view)` call (light and dark). Refresh with `go test -run <Name> -update .`
 - Run `go vet ./... && go test ./...` before reporting done.
-- Snapshots assume the default test order. MyGo shares its path-mask cache across a process, and its own chevrons are not snapped, so `-shuffle` can shift a few pixels (docs/upstream.md).
+- Each snapshot test renders in a fresh child process (snapshot_test.go), because MyGo's path-mask cache is process-wide. Order and neighbours never change pixels; `-run <Name> -update` is safe.
 
 ## Gallery page
 

docs/upstream.md

@@ -35,4 +35,4 @@ PRD 第 2 节要求公开接口不足时向上游提交扩展。以下缺口在
 | `SelectBase` 高亮无设置接口 | `ui/base.go:260` | Select |
 | `TextInputBase` 无只读开关与占位符 | `ui/editor.go:139,734,811` | TextInput 只读模式、CommandPalette |
 | `ui.Stepper` 禁用时仍可步进，固定 9 位取整 | `ui/indicators.go:216`、`ui/widgets.go` `snap` | NumberInput（已改为自绘） |
-| 路径遮罩缓存按四分之一像素取键，进程内共享 | `ui/path.go:189-210` | 自绘路径以 `internal.Snap` 对齐；MyGo 自绘的箭头（Outline 等）无法对齐，快照依赖测试运行顺序，`-shuffle` 下 TreeTable 快照会出现 6 像素差异 |
+| 路径遮罩缓存按四分之一像素取键，进程内共享 | `ui/path.go:189-210` | 自绘路径以 `internal.Snap` 对齐；MyGo 自绘的箭头（Outline 等）无法对齐；快照测试因此在独立子进程中渲染，结果与测试顺序无关 |

snapshot_test.go

@@ -6,9 +6,12 @@ import (
 	"image"
 	"image/png"
 	"os"
+	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strings"
+	"sync"
 	"testing"
 
 	"github.com/egoist/mygo/ui"
@@ -22,9 +25,18 @@ func snapshot(t *testing.T, name string, w, h int, view func(c *ui.Context)) {
 	snapshotAfter(t, name, w, h, view, nil)
 }
 
+// snapshotChild marks a process that renders snapshots itself.
+const snapshotChild = "MUJICA_SNAPSHOT_CHILD"
+
+var snapshotRan sync.Map
+
 // snapshotAfter is snapshot with drive sending input before the capture.
 func snapshotAfter(t *testing.T, name string, w, h int, view func(c *ui.Context), drive func(tt *ui.Tester)) {
 	t.Helper()
+	if os.Getenv(snapshotChild) == "" {
+		snapshotInChild(t)
+		return
+	}
 	for _, m := range []Mode{Light, Dark} {
 		ready := false
 		tt := ui.NewTester(func(c *ui.Context) {
@@ -64,6 +76,27 @@ func snapshotAfter(t *testing.T, name string, w, h int, view func(c *ui.Context)
 	}
 }
 
+// snapshotInChild reruns the test alone in a fresh process; MyGo's path cache is process-wide.
+func snapshotInChild(t *testing.T) {
+	t.Helper()
+	if _, done := snapshotRan.LoadOrStore(t.Name(), true); done {
+		return
+	}
+	parts := strings.Split(t.Name(), "/")
+	for i, p := range parts {
+		parts[i] = "^" + regexp.QuoteMeta(p) + "$"
+	}
+	args := []string{"-test.run=" + strings.Join(parts, "/"), "-test.count=1"}
+	if *update {
+		args = append(args, "-update")
+	}
+	cmd := exec.Command(os.Args[0], args...)
+	cmd.Env = append(os.Environ(), snapshotChild+"=1")
+	if out, err := cmd.CombinedOutput(); err != nil {
+		t.Fatalf("snapshot child failed: %v\n%s", err, out)
+	}
+}
+
 func encode(t *testing.T, img image.Image) []byte {
 	t.Helper()
 	var b bytes.Buffer