Add RTL rendering support to Markdown (#24816)

Support RTL content in Markdown:


![image](https://github.com/go-gitea/gitea/assets/115237/dedb1b0c-2f05-40dc-931a-0d9dc81f7c97)

Example document:
https://try.gitea.io/silverwind/symlink-test/src/branch/master/bidi-text.md
Same on GitHub:
https://github.com/silverwind/symlink-test/blob/master/bidi-text.md

`dir=auto` enables a browser heuristic that sets the text direction
automatically. It is the only way to get automatic text direction.

Ref: https://codeberg.org/Codeberg/Community/issues/1021

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/markup/html.go

@@ -630,7 +630,7 @@ func mentionProcessor(ctx *RenderContext, node *html.Node) {
 		}
 		mentionedUsername := mention[1:]
 
-		if processorHelper.IsUsernameMentionable != nil && processorHelper.IsUsernameMentionable(ctx.Ctx, mentionedUsername) {
+		if DefaultProcessorHelper.IsUsernameMentionable != nil && DefaultProcessorHelper.IsUsernameMentionable(ctx.Ctx, mentionedUsername) {
 			replaceContent(node, loc.Start, loc.End, createLink(util.URLJoin(setting.AppURL, mentionedUsername), mention, "mention"))
 			node = node.NextSibling.NextSibling
 		} else {

modules/markup/markdown/goldmark.go

@@ -47,6 +47,12 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 		tocMode = rc.TOC
 	}
 
+	applyElementDir := func(n ast.Node) {
+		if markup.DefaultProcessorHelper.ElementDir != "" {
+			n.SetAttributeString("dir", []byte(markup.DefaultProcessorHelper.ElementDir))
+		}
+	}
+
 	attentionMarkedBlockquotes := make(container.Set[*ast.Blockquote])
 	_ = ast.Walk(node, func(n ast.Node, entering bool) (ast.WalkStatus, error) {
 		if !entering {
@@ -69,6 +75,9 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 				header.ID = util.BytesToReadOnlyString(id.([]byte))
 			}
 			tocList = append(tocList, header)
+			applyElementDir(v)
+		case *ast.Paragraph:
+			applyElementDir(v)
 		case *ast.Image:
 			// Images need two things:
 			//
@@ -171,6 +180,7 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 					v.AppendChild(v, newChild)
 				}
 			}
+			applyElementDir(v)
 		case *ast.Text:
 			if v.SoftLineBreak() && !v.HardLineBreak() {
 				renderMetas := pc.Get(renderMetasKey).(map[string]string)

modules/markup/renderer.go

@@ -30,14 +30,16 @@ const (
 
 type ProcessorHelper struct {
 	IsUsernameMentionable func(ctx context.Context, username string) bool
+
+	ElementDir string // the direction of the elements, eg: "ltr", "rtl", "auto", default to no direction attribute
 }
 
-var processorHelper ProcessorHelper
+var DefaultProcessorHelper ProcessorHelper
 
 // Init initialize regexps for markdown parsing
 func Init(ph *ProcessorHelper) {
 	if ph != nil {
-		processorHelper = *ph
+		DefaultProcessorHelper = *ph
 	}
 
 	NewSanitizer()