fix(highlight): highlight Svelte components (#3487)

* fix(highlight): highlight Svelte components

`.svelte` had no entry in the extension table, so `highlightCode` fell through
to one unstyled token per line. That is the same table the file pane, the diff
tokens, markdown fences and the web editor all resolve through, so a component
rendered as plain text everywhere in the app.

Register `@replit/codemirror-lang-svelte`. Its `svelteLanguage` already wires
the mixed parsing for the `<script>` and `<style>` blocks, so the script gets
JS/TS roles, the style block gets CSS roles, and the template gets tag,
directive and mustache roles. It comes from the same publisher as the
`@replit/codemirror-lang-csharp` entry already in this package, and only the
grammar itself is declared: its CodeMirror peers resolve the same way the C#
grammar's peers already do.

* test(server): cover Svelte diff highlighting

The review surface in the issue is the diff, and the daemon is what tokenizes
it (`diff-highlighter.ts` calls `highlightCode`), so the regression belongs
here too: script, template block and markup roles on a `.svelte` hunk.

packages/highlight/package.json

@@ -47,6 +47,7 @@
     "@lezer/yaml": "^1.0.4",
     "@replit/codemirror-lang-csharp": "^6.2.0",
     "@replit/codemirror-lang-nix": "^6.0.1",
+    "@replit/codemirror-lang-svelte": "^6.0.0",
     "lezer-elixir": "^1.1.2"
   },
   "devDependencies": {

packages/highlight/src/__tests__/highlighter.test.ts

@@ -86,6 +86,38 @@ describe("highlightCode", () => {
     expect(stringToken?.style).toBe("string");
   });
 
+  it("highlights Svelte components across script, markup, and style", () => {
+    const code = [
+      '<script lang="ts">',
+      "  let count: number = 1;",
+      "</script>",
+      "",
+      "{#if count > 0}",
+      '  <span class="badge">{count}</span>',
+      "{/if}",
+      "",
+      "<style>",
+      "  .badge {",
+      "    color: red;",
+      "  }",
+      "</style>",
+    ].join("\n");
+    const result = highlightCode(code, "Counter.svelte");
+
+    expect(result[1].find((t) => t.text === "let")?.style).toBe("keyword");
+    expect(result[1].find((t) => t.text === "number")?.style).toBe("type");
+
+    expect(result[4].find((t) => t.text === "if")?.style).toBe("keyword");
+    expect(result[4].find((t) => t.text === "count")?.style).toBe("variable");
+
+    expect(result[5].find((t) => t.text === "span")?.style).toBe("tag");
+    expect(result[5].find((t) => t.text === "class")?.style).toBe("attribute");
+    expect(result[5].find((t) => t.text === "count")?.style).toBe("variable");
+
+    expect(result[9].find((t) => t.text === "badge")?.style).toBe("class");
+    expect(result[10].find((t) => t.text === "color")?.style).toBe("property");
+  });
+
   it("highlights TSX code with correct dialect", () => {
     const code = 'const el = <div className="test">hello</div>;';
     const result = highlightCode(code, "test.tsx");

packages/highlight/src/__tests__/parsers.test.ts

@@ -23,6 +23,7 @@ describe("isLanguageSupported", () => {
     expect(isLanguageSupported("test.cs")).toBe(true);
     expect(isLanguageSupported("test.nix")).toBe(true);
     expect(isLanguageSupported("test.ex")).toBe(true);
+    expect(isLanguageSupported("Counter.svelte")).toBe(true);
   });
 
   it("returns false for unsupported file extensions", () => {
@@ -63,6 +64,7 @@ describe("getSupportedExtensions", () => {
     expect(extensions).toContain("cs");
     expect(extensions).toContain("nix");
     expect(extensions).toContain("json");
+    expect(extensions).toContain("svelte");
   });
 });
 

packages/highlight/src/parsers.ts

@@ -16,6 +16,7 @@ import { parser as xmlParser } from "@lezer/xml";
 import { parser as yamlParser } from "@lezer/yaml";
 import { csharpLanguage } from "@replit/codemirror-lang-csharp";
 import { nixLanguage } from "@replit/codemirror-lang-nix";
+import { svelteLanguage } from "@replit/codemirror-lang-svelte";
 import { parser as elixirParser } from "lezer-elixir";
 import type { Parser } from "@lezer/common";
 
@@ -49,6 +50,8 @@ const languagesByExtension: Record<string, Language> = {
   // HTML
   html: language(htmlParser),
   htm: language(htmlParser),
+  // Svelte
+  svelte: svelteLanguage,
   // XML
   xml: language(xmlParser),
   // Java

packages/server/src/server/utils/diff-highlighter.test.ts

@@ -160,6 +160,21 @@ index 1234567..abcdefg 100644
  </config>
 `;
 
+const SVELTE_DIFF = `diff --git a/Counter.svelte b/Counter.svelte
+index 1234567..abcdefg 100644
+--- a/Counter.svelte
++++ b/Counter.svelte
+@@ -1,7 +1,7 @@
+ <script lang="ts">
+-  let count: number = 1;
++  let count: number = 2;
+ </script>
+
+ {#if count > 0}
+   <span class="badge">{count}</span>
+ {/if}
+`;
+
 describe("parseDiff", () => {
   it("parses a simple diff with one hunk", () => {
     const files = parseDiff(SIMPLE_DIFF);
@@ -471,6 +486,33 @@ describe("highlightDiffFromHunks", () => {
     expect(addedLine!.tokens!.some((t) => t.text === "2" && t.style === "number")).toBe(true);
   });
 
+  it("adds syntax highlighting tokens to Svelte components", () => {
+    const files = parseDiff(SVELTE_DIFF);
+    const highlighted = highlightDiffFromHunks(files[0]);
+    const hunk = highlighted.hunks[0];
+
+    const scriptLine = hunk.lines[1];
+    expect(scriptLine.tokens).toBeDefined();
+    expect(scriptLine.tokens!.some((t) => t.text === "script" && t.style === "tag")).toBe(true);
+
+    const addedLine = hunk.lines.find(
+      (line) => line.type === "add" && line.content.includes("count"),
+    );
+    expect(addedLine?.tokens).toBeDefined();
+    expect(addedLine!.tokens!.some((t) => t.text === "let" && t.style === "keyword")).toBe(true);
+    expect(addedLine!.tokens!.some((t) => t.text === "number" && t.style === "type")).toBe(true);
+    expect(addedLine!.tokens!.some((t) => t.text === "2" && t.style === "number")).toBe(true);
+
+    const blockLine = hunk.lines.find((line) => line.content.includes("{#if"));
+    expect(blockLine?.tokens).toBeDefined();
+    expect(blockLine!.tokens!.some((t) => t.text === "if" && t.style === "keyword")).toBe(true);
+    expect(blockLine!.tokens!.some((t) => t.text === "count" && t.style === "variable")).toBe(true);
+
+    const markupLine = hunk.lines.find((line) => line.content.includes("<span"));
+    expect(markupLine?.tokens).toBeDefined();
+    expect(markupLine!.tokens!.some((t) => t.text === "span" && t.style === "tag")).toBe(true);
+  });
+
   it("adds syntax highlighting tokens to Objective-C file extensions", () => {
     const files = parseDiff(OBJECTIVE_C_DIFF);
     const highlighted = highlightDiffFromHunks(files[0]);