Add color previews in markdown (#21474)

* Resolves #3047

Every time a color code will be in \`backticks`, a cute little color
preview will pop up


[Inspiration](https://docs.github.com/en/get-started/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#supported-color-models)

#### Before

![image](https://user-images.githubusercontent.com/20454870/196631524-298afbbf-d2c8-4018-92a5-0393a693d850.png)

#### After

![image](https://user-images.githubusercontent.com/20454870/196631397-36c561e4-08f5-465a-a36e-76084e30b08a.png)

Signed-off-by: Yarden Shoham <hrsi88@gmail.com>
Co-authored-by: KN4CK3R <admin@oldschoolhack.me>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

4 files changed, 135 insertions, 2 deletions

diff --git a/modules/markup/markdown/ast.go b/modules/markup/markdown/ast.go
index 5191d94cdd..c82d5e5e73 100644
--- a/modules/markup/markdown/ast.go
+++ b/modules/markup/markdown/ast.go
@@ -144,3 +144,39 @@ func IsIcon(node ast.Node) bool {
 	_, ok := node.(*Icon)
 	return ok
 }
+
+// ColorPreview is an inline for a color preview
+type ColorPreview struct {
+	ast.BaseInline
+	Color []byte
+}
+
+// Dump implements Node.Dump.
+func (n *ColorPreview) Dump(source []byte, level int) {
+	m := map[string]string{}
+	m["Color"] = string(n.Color)
+	ast.DumpHelper(n, source, level, m, nil)
+}
+
+// KindColorPreview is the NodeKind for ColorPreview
+var KindColorPreview = ast.NewNodeKind("ColorPreview")
+
+// Kind implements Node.Kind.
+func (n *ColorPreview) Kind() ast.NodeKind {
+	return KindColorPreview
+}
+
+// NewColorPreview returns a new Span node.
+func NewColorPreview(color []byte) *ColorPreview {
+	return &ColorPreview{
+		BaseInline: ast.BaseInline{},
+		Color:      color,
+	}
+}
+
+// IsColorPreview returns true if the given node implements the ColorPreview interface,
+// otherwise false.
+func IsColorPreview(node ast.Node) bool {
+	_, ok := node.(*ColorPreview)
+	return ok
+}
diff --git a/modules/markup/markdown/goldmark.go b/modules/markup/markdown/goldmark.go
index 8417019ddb..1a36681366 100644
--- a/modules/markup/markdown/goldmark.go
+++ b/modules/markup/markdown/goldmark.go
@@ -16,6 +16,7 @@ import (
 	"code.gitea.io/gitea/modules/setting"
 	giteautil "code.gitea.io/gitea/modules/util"
 
+	"github.com/microcosm-cc/bluemonday/css"
 	"github.com/yuin/goldmark/ast"
 	east "github.com/yuin/goldmark/extension/ast"
 	"github.com/yuin/goldmark/parser"
@@ -178,6 +179,11 @@ func (g *ASTTransformer) Transform(node *ast.Document, reader text.Reader, pc pa
 					v.SetHardLineBreak(setting.Markdown.EnableHardLineBreakInDocuments)
 				}
 			}
+		case *ast.CodeSpan:
+			colorContent := n.Text(reader.Source())
+			if css.ColorHandler(strings.ToLower(string(colorContent))) {
+				v.AppendChild(v, NewColorPreview(colorContent))
+			}
 		}
 		return ast.WalkContinue, nil
 	})
@@ -266,10 +272,43 @@ func (r *HTMLRenderer) RegisterFuncs(reg renderer.NodeRendererFuncRegisterer) {
 	reg.Register(KindDetails, r.renderDetails)
 	reg.Register(KindSummary, r.renderSummary)
 	reg.Register(KindIcon, r.renderIcon)
+	reg.Register(ast.KindCodeSpan, r.renderCodeSpan)
 	reg.Register(KindTaskCheckBoxListItem, r.renderTaskCheckBoxListItem)
 	reg.Register(east.KindTaskCheckBox, r.renderTaskCheckBox)
 }
 
+// renderCodeSpan renders CodeSpan elements (like goldmark upstream does) but also renders ColorPreview elements.
+// See #21474 for reference
+func (r *HTMLRenderer) renderCodeSpan(w util.BufWriter, source []byte, n ast.Node, entering bool) (ast.WalkStatus, error) {
+	if entering {
+		if n.Attributes() != nil {
+			_, _ = w.WriteString("<code")
+			html.RenderAttributes(w, n, html.CodeAttributeFilter)
+			_ = w.WriteByte('>')
+		} else {
+			_, _ = w.WriteString("<code>")
+		}
+		for c := n.FirstChild(); c != nil; c = c.NextSibling() {
+			switch v := c.(type) {
+			case *ast.Text:
+				segment := v.Segment
+				value := segment.Value(source)
+				if bytes.HasSuffix(value, []byte("\n")) {
+					r.Writer.RawWrite(w, value[:len(value)-1])
+					r.Writer.RawWrite(w, []byte(" "))
+				} else {
+					r.Writer.RawWrite(w, value)
+				}
+			case *ColorPreview:
+				_, _ = w.WriteString(fmt.Sprintf(`<span class="color-preview" style="background-color: %v"></span>`, string(v.Color)))
+			}
+		}
+		return ast.WalkSkipChildren, nil
+	}
+	_, _ = w.WriteString("</code>")
+	return ast.WalkContinue, nil
+}
+
 func (r *HTMLRenderer) renderDocument(w util.BufWriter, source []byte, node ast.Node, entering bool) (ast.WalkStatus, error) {
 	n := node.(*ast.Document)
 
diff --git a/modules/markup/markdown/markdown_test.go b/modules/markup/markdown/markdown_test.go
index 49ed3d75d6..12c6288c24 100644
--- a/modules/markup/markdown/markdown_test.go
+++ b/modules/markup/markdown/markdown_test.go
@@ -429,6 +429,61 @@ func TestRenderEmojiInLinks_Issue12331(t *testing.T) {
 	assert.Equal(t, expected, res)
 }
 
+func TestColorPreview(t *testing.T) {
+	const nl = "\n"
+	positiveTests := []struct {
+		testcase string
+		expected string
+	}{
+		{ // hex
+			"`#FF0000`",
+			`<p><code>#FF0000<span class="color-preview" style="background-color: #FF0000"></span></code></p>` + nl,
+		},
+		{ // rgb
+			"`rgb(16, 32, 64)`",
+			`<p><code>rgb(16, 32, 64)<span class="color-preview" style="background-color: rgb(16, 32, 64)"></span></code></p>` + nl,
+		},
+		{ // short hex
+			"This is the color white `#000`",
+			`<p>This is the color white <code>#000<span class="color-preview" style="background-color: #000"></span></code></p>` + nl,
+		},
+		{ // hsl
+			"HSL stands for hue, saturation, and lightness. An example: `hsl(0, 100%, 50%)`.",
+			`<p>HSL stands for hue, saturation, and lightness. An example: <code>hsl(0, 100%, 50%)<span class="color-preview" style="background-color: hsl(0, 100%, 50%)"></span></code>.</p>` + nl,
+		},
+		{ // uppercase hsl
+			"HSL stands for hue, saturation, and lightness. An example: `HSL(0, 100%, 50%)`.",
+			`<p>HSL stands for hue, saturation, and lightness. An example: <code>HSL(0, 100%, 50%)<span class="color-preview" style="background-color: HSL(0, 100%, 50%)"></span></code>.</p>` + nl,
+		},
+	}
+
+	for _, test := range positiveTests {
+		res, err := RenderString(&markup.RenderContext{}, test.testcase)
+		assert.NoError(t, err, "Unexpected error in testcase: %q", test.testcase)
+		assert.Equal(t, test.expected, res, "Unexpected result in testcase %q", test.testcase)
+
+	}
+
+	negativeTests := []string{
+		// not a color code
+		"`FF0000`",
+		// inside a code block
+		"```javascript" + nl + `const red = "#FF0000";` + nl + "```",
+		// no backticks
+		"rgb(166, 32, 64)",
+		// typo
+		"`hsI(0, 100%, 50%)`",
+		// looks like a color but not really
+		"`hsl(40, 60, 80)`",
+	}
+
+	for _, test := range negativeTests {
+		res, err := RenderString(&markup.RenderContext{}, test)
+		assert.NoError(t, err, "Unexpected error in testcase: %q", test)
+		assert.NotContains(t, res, `<span class="color-preview" style="background-color: `, "Unexpected result in testcase %q", test)
+	}
+}
+
 func TestMathBlock(t *testing.T) {
 	const nl = "\n"
 	testcases := []struct {
diff --git a/modules/markup/sanitizer.go b/modules/markup/sanitizer.go
index 807a8a7892..ff7165c131 100644
--- a/modules/markup/sanitizer.go
+++ b/modules/markup/sanitizer.go
@@ -55,6 +55,9 @@ func createDefaultPolicy() *bluemonday.Policy {
 	// For JS code copy and Mermaid loading state
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^code-block( is-loading)?$`)).OnElements("pre")
 
+	// For color preview
+	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^color-preview$`)).OnElements("span")
+
 	// For Chroma markdown plugin
 	policy.AllowAttrs("class").Matching(regexp.MustCompile(`^(chroma )?language-[\w-]+( display)?( is-loading)?$`)).OnElements("code")
 
@@ -88,8 +91,8 @@ func createDefaultPolicy() *bluemonday.Policy {
 	// Allow 'style' attribute on text elements.
 	policy.AllowAttrs("style").OnElements("span", "p")
 
-	// Allow 'color' property for the style attribute on text elements.
-	policy.AllowStyles("color").OnElements("span", "p")
+	// Allow 'color' and 'background-color' properties for the style attribute on text elements.
+	policy.AllowStyles("color", "background-color").OnElements("span", "p")
 
 	// Allow generally safe attributes
 	generalSafeAttrs := []string{