fix scratchpad routing by caller project

internal/app/project_registry_test.go

@@ -98,3 +98,65 @@ func TestSwitchProjectPreservesProjectProcessTrees(t *testing.T) {
 		t.Fatalf("switching back should preserve both project process trees")
 	}
 }
+
+func TestProjectRegistryScratchpadsRouteByCallerProject(t *testing.T) {
+	t.Setenv("XDG_DATA_HOME", t.TempDir())
+	t.Setenv("XDG_CONFIG_HOME", t.TempDir())
+
+	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
+
+	reg := newProjectRegistry(preset.Set{}, defaultSettings(), nil, 80, 24)
+	defer reg.Shutdown()
+
+	projectA, err := reg.Open(ctx, t.TempDir())
+	if err != nil {
+		t.Fatalf("open project A: %v", err)
+	}
+	projectB, err := reg.Open(ctx, t.TempDir())
+	if err != nil {
+		t.Fatalf("open project B: %v", err)
+	}
+
+	a, err := projectA.Session.Spawn(SpawnSpec{
+		Kind: KindCommand,
+		Argv: []string{"sh", "-c", "trap 'exit 0' TERM; while :; do sleep 1; done"},
+		Name: "a-caller",
+	}, 80, 24)
+	if err != nil {
+		t.Fatalf("spawn project A caller: %v", err)
+	}
+	b, err := projectB.Session.Spawn(SpawnSpec{
+		Kind: KindCommand,
+		Argv: []string{"sh", "-c", "trap 'exit 0' TERM; while :; do sleep 1; done"},
+		Name: "b-caller",
+	}, 80, 24)
+	if err != nil {
+		t.Fatalf("spawn project B caller: %v", err)
+	}
+	t.Cleanup(func() {
+		_ = projectA.Session.Kill(a.ID, syscall.SIGTERM)
+		_ = projectB.Session.Kill(b.ID, syscall.SIGTERM)
+	})
+	waitUntilLive(t, a)
+	waitUntilLive(t, b)
+
+	if _, err := reg.ScratchpadWrite(a.ID, "note.md", "project A", ""); err != nil {
+		t.Fatalf("write project A scratchpad: %v", err)
+	}
+	if _, err := reg.ScratchpadWrite(b.ID, "note.md", "project B", ""); err != nil {
+		t.Fatalf("write project B scratchpad: %v", err)
+	}
+
+	gotA, _, err := reg.ScratchpadRead(a.ID, "note.md")
+	if err != nil {
+		t.Fatalf("read project A scratchpad: %v", err)
+	}
+	gotB, _, err := reg.ScratchpadRead(b.ID, "note.md")
+	if err != nil {
+		t.Fatalf("read project B scratchpad: %v", err)
+	}
+	if gotA != "project A" || gotB != "project B" {
+		t.Fatalf("scratchpad routing leaked between projects: A=%q B=%q", gotA, gotB)
+	}
+}