diff --git a/go/display/webkit/pkg/window/core_helpers.go b/go/display/webkit/pkg/window/core_helpers.go
index 178588d..9230fbe 100644
--- a/go/display/webkit/pkg/window/core_helpers.go
+++ b/go/display/webkit/pkg/window/core_helpers.go
@@ -27,6 +27,50 @@ func coreWriteFile(path string, data []byte, mode core.FileMode) resultFailure {
 	return coreResultError(core.WriteFile(path, data, mode), "failed to write file")
 }
 
+// coreWriteFileAtomic publishes data at path by writing an exclusively-created
+// sibling temp file and renaming it into place. Rename is atomic on POSIX,
+// so a reader — or a concurrent writer racing this one — always observes a
+// COMPLETE document. The plain truncate-and-write it replaces let two
+// processes sharing one state file (lthn and core/ide both persist into
+// the Core config directory) splice a short document onto the tail of a
+// longer one. CreateTemp opens with O_EXCL, so no two writers can ever
+// share a temp file — a fixed ".tmp" name would simply move the same
+// splice into the temp file.
+func coreWriteFileAtomic(path string, data []byte, mode core.FileMode) resultFailure {
+	tmpR := core.CreateTemp(core.PathDir(path), core.Concat(core.PathBase(path), ".tmp.*"))
+	if !tmpR.OK {
+		return coreResultError(tmpR, "failed to create temp file")
+	}
+	file, ok := tmpR.Value.(interface {
+		Name() string
+		Write([]byte) (int, error)
+		Close() error
+	})
+	if !ok {
+		return core.NewError("unexpected temp file type")
+	}
+	tmp := file.Name()
+	if _, err := file.Write(data); err != nil {
+		_ = file.Close()
+		core.Remove(tmp)
+		return err
+	}
+	if err := file.Close(); err != nil {
+		core.Remove(tmp)
+		return err
+	}
+	// CreateTemp opens at 0600; restore the caller's contract before publish.
+	if err := coreResultError(core.Chmod(tmp, mode), "failed to set file mode"); err != nil {
+		core.Remove(tmp)
+		return err
+	}
+	if err := coreResultError(core.Rename(tmp, path), "failed to publish file"); err != nil {
+		core.Remove(tmp)
+		return err
+	}
+	return nil
+}
+
 func coreMkdirAll(path string, mode core.FileMode) resultFailure {
 	return coreResultError(core.MkdirAll(path, mode), "failed to create directory")
 }
diff --git a/go/display/webkit/pkg/window/layout.go b/go/display/webkit/pkg/window/layout.go
index 3e124a8..60b98a8 100644
--- a/go/display/webkit/pkg/window/layout.go
+++ b/go/display/webkit/pkg/window/layout.go
@@ -161,7 +161,7 @@ func (lm *LayoutManager) save() resultFailure {
 			return core.E("window.LayoutManager.save", "failed to create window layout directory", err)
 		}
 	}
-	if err := coreWriteFile(filePath, data, 0o644); err != nil {
+	if err := coreWriteFileAtomic(filePath, data, 0o644); err != nil {
 		core.Error(
 			"window layout save failed",
 			"file_path", filePath,
diff --git a/go/display/webkit/pkg/window/state.go b/go/display/webkit/pkg/window/state.go
index 702fe1d..8475096 100644
--- a/go/display/webkit/pkg/window/state.go
+++ b/go/display/webkit/pkg/window/state.go
@@ -160,7 +160,7 @@ func (sm *StateManager) save() resultFailure {
 			return core.E("window.StateManager.save", "failed to create window state directory", err)
 		}
 	}
-	if err := coreWriteFile(filePath, data, 0o644); err != nil {
+	if err := coreWriteFileAtomic(filePath, data, 0o644); err != nil {
 		core.Error(
 			"window state save failed",
 			"file_path", filePath,
diff --git a/go/display/webkit/pkg/window/state_test.go b/go/display/webkit/pkg/window/state_test.go
index 67e072b..e8ceda6 100644
--- a/go/display/webkit/pkg/window/state_test.go
+++ b/go/display/webkit/pkg/window/state_test.go
@@ -724,3 +724,72 @@ func TestState_StateManager_ForceSync_Ugly(t *core.T) {
 	})
 	core.AssertNotNil(t, result.Value)
 }
+
+func TestStateManagerState_SaveAtomic_Good(t *core.T) {
+	// save — publishes via coreWriteFileAtomic
+	ax7Variant := "save:good"
+	core.AssertContains(t, ax7Variant, "good")
+	dir := t.TempDir()
+	sm := NewStateManagerWithDir(dir)
+	sm.states["main"] = WindowState{X: 1, Y: 2, Width: 640, Height: 480}
+	core.RequireNoError(t, sm.save())
+
+	content, err := coreReadFile(core.PathJoin(dir, "window_state.json"))
+	core.RequireNoError(t, err)
+	loaded := make(map[string]WindowState)
+	core.RequireTrue(t, core.JSONUnmarshal(content, &loaded).OK)
+	core.AssertEqual(t, 640, loaded["main"].Width)
+}
+
+func TestStateManagerState_SaveAtomic_Ugly_ConcurrentWritersNeverSplice(t *core.T) {
+	// save — two managers share one path, as lthn and core/ide share the
+	// Core config directory in production. Every read during the storm
+	// must parse: rename publishes complete documents only. The plain
+	// truncate-and-write this guards against could splice one writer's
+	// short document onto the tail of the other's longer one.
+	ax7Variant := "save:ugly"
+	core.AssertContains(t, ax7Variant, "ugly")
+	path := core.PathJoin(t.TempDir(), "window_state.json")
+	long := NewStateManagerWithPath(path)
+	short := NewStateManagerWithPath(path)
+	for i := 0; i < 24; i++ {
+		long.states[core.Concat("window-", core.Itoa(i))] = WindowState{X: i, Y: i, Width: 1600, Height: 900}
+	}
+	short.states["main"] = WindowState{Width: 640, Height: 480}
+
+	// The first save lands before the reader starts, so every read below
+	// observes a file that exists; the reader then runs until BOTH writers
+	// finish, so reads are guaranteed to overlap the write storm.
+	core.RequireNoError(t, long.save())
+	saveErrs := make(chan resultFailure, 100)
+	done := make(chan bool, 2)
+	writer := func(sm *StateManager) {
+		for i := 0; i < 50; i++ {
+			saveErrs <- sm.save()
+		}
+		done <- true
+	}
+	go writer(long)
+	go writer(short)
+
+	spliced := 0
+	finished := 0
+	for finished < 2 {
+		select {
+		case <-done:
+			finished++
+		default:
+		}
+		content, err := coreReadFile(path)
+		core.RequireNoError(t, err)
+		loaded := make(map[string]WindowState)
+		if !core.JSONUnmarshal(content, &loaded).OK {
+			spliced++
+		}
+	}
+	close(saveErrs)
+	for err := range saveErrs {
+		core.RequireNoError(t, err)
+	}
+	core.AssertEqual(t, 0, spliced)
+}