Make the shortcode template lookup for output formats stable

Fixes #7774

hugolib/shortcode_test.go

@@ -1347,3 +1347,49 @@ title: "No Inner!"
 	err := b.BuildE(BuildCfg{})
 	b.Assert(err.Error(), qt.Contains, `failed to extract shortcode: shortcode "noinner" has no .Inner, yet a closing tag was provided`)
 }
+
+func TestShortcodeStableOutputFormatTemplates(t *testing.T) {
+	t.Parallel()
+
+	for i := 0; i < 5; i++ {
+
+		b := newTestSitesBuilder(t)
+
+		const numPages = 10
+
+		for i := 0; i < numPages; i++ {
+			b.WithContent(fmt.Sprintf("page%d.md", i), `---
+title: "Page"
+outputs: ["html", "css", "csv", "json"]
+---
+{{< myshort >}}
+
+`)
+		}
+
+		b.WithTemplates(
+			"_default/single.html", "{{ .Content }}",
+			"_default/single.css", "{{ .Content }}",
+			"_default/single.csv", "{{ .Content }}",
+			"_default/single.json", "{{ .Content }}",
+			"shortcodes/myshort.html", `Short-HTML`,
+			"shortcodes/myshort.csv", `Short-CSV`,
+		)
+
+		b.Build(BuildCfg{})
+
+		//helpers.PrintFs(b.Fs.Destination, "public", os.Stdout)
+
+		for i := 0; i < numPages; i++ {
+			b.AssertFileContent(fmt.Sprintf("public/page%d/index.html", i), "Short-HTML")
+			b.AssertFileContent(fmt.Sprintf("public/page%d/index.csv", i), "Short-CSV")
+			b.AssertFileContent(fmt.Sprintf("public/page%d/index.json", i), "Short-HTML")
+
+		}
+
+		for i := 0; i < numPages; i++ {
+			b.AssertFileContent(fmt.Sprintf("public/page%d/styles.css", i), "Short-HTML")
+		}
+
+	}
+}

tpl/tplimpl/template.go

@@ -19,6 +19,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"regexp"
+	"sort"
 	"strings"
 	"sync"
 	"time"
@@ -814,6 +815,24 @@ func (t *templateHandler) postTransform() error {
 		}
 	}
 
+	for _, v := range t.shortcodes {
+		sort.Slice(v.variants, func(i, j int) bool {
+			v1, v2 := v.variants[i], v.variants[j]
+			name1, name2 := v1.ts.Name(), v2.ts.Name()
+			isHTMl1, isHTML2 := strings.HasSuffix(name1, "html"), strings.HasSuffix(name2, "html")
+
+			// There will be a weighted selection later, but make
+			// sure these are sorted to get a stable selection for
+			// output formats missing specific templates.
+			// Prefer HTML.
+			if isHTMl1 || isHTML2 && !(isHTMl1 && isHTML2) {
+				return isHTMl1
+			}
+
+			return name1 < name2
+		})
+	}
+
 	return nil
 }