Keep explanation origins scoped to the query

facade_container_origin_test.go

@@ -0,0 +1,41 @@
+package configkit_test
+
+import (
+	"testing"
+	"testing/fstest"
+
+	configkit "gitea.maximumdirect.net/eric/ConfigKit"
+)
+
+func TestOverlayExplanationUsesQueryScopedOrigin(t *testing.T) {
+	t.Parallel()
+	filesystem := configkit.NewFilesystem("origin", fstest.MapFS{
+		"main.yml":         {Data: []byte("a: base\ngroup: {x: X, y: Y}\n")},
+		"overlays/dev.yml": {Data: []byte("a: overlay\ngroup: {x: O}\n")},
+	})
+	result, err := configkit.Load[map[string]any](filesystem.File("main.yml"), configkit.Options[map[string]any]{Overlay: "dev"})
+	if err != nil {
+		t.Fatal(err)
+	}
+	for _, test := range []struct {
+		path         configkit.Path
+		line, column int
+	}{
+		{configkit.Keys(), 1, 1},
+		{configkit.Keys("group"), 2, 8},
+		{configkit.Keys("group", "x"), 2, 12},
+	} {
+		explanation, found := result.Explain(test.path)
+		if !found || len(explanation.History) == 0 {
+			t.Fatalf("missing explanation for %s", test.path)
+		}
+		origin := explanation.History[len(explanation.History)-1].Origin
+		if origin.Layer != configkit.LayerOverlay || len(origin.Locations) != 1 {
+			t.Fatalf("overlay origin for %s = %#v", test.path, origin)
+		}
+		location := origin.Locations[0]
+		if location.SourceID != "source-2" || location.Line != test.line || location.Column != test.column {
+			t.Fatalf("overlay location for %s = %#v; want line %d column %d", test.path, location, test.line, test.column)
+		}
+	}
+}