Fix cascade with overlapping sections

Fixes #12465

hugolib/cascade_test.go

@@ -876,3 +876,44 @@ Background: {{ .Params.background }}|
 		b.AssertFileContent("public/p1/index.html", "Background: yosemite.jpg")
 	}
 }
+
+// Issue #12465.
+func TestCascadeOverlap(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['home','rss','sitemap','taxonomy','term']
+-- layouts/_default/list.html --
+{{ .Title }}
+-- layouts/_default/single.html --
+{{ .Title }}
+-- content/s/_index.md --
+---
+title: s
+cascade:
+  _build:
+    render: never
+---
+-- content/s/p1.md --
+---
+title: p1
+---
+-- content/sx/_index.md --
+---
+title: sx
+---
+-- content/sx/p2.md --
+---
+title: p2
+---
+`
+
+	b := Test(t, files)
+
+	b.AssertFileExists("public/s/index.html", false)
+	b.AssertFileExists("public/s/p1/index.html", false)
+
+	b.AssertFileExists("public/sx/index.html", true)    // failing
+	b.AssertFileExists("public/sx/p2/index.html", true) // failing
+}