From d155cfba0563489210aa2766fd29ea1af807f795 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jorge=20Albarrac=C3=ADn?= <albarracinj.dev@gmail.com>
Date: Wed, 28 Oct 2026 05:10:23 -0300
Subject: [PATCH] feat(editor): syntax highlighting for Glimmer .gjs/.gts files
 (#13236)

Register glimmer-js and glimmer-ts Monaco languages that embed
JavaScript/TypeScript as the background script and tokenize each
<template> block as a Handlebars island. Embed entry is bounded by the
shared line budget so long lines cannot blow the Monarch recursion
limit. Mobile maps the new ids to javascript/typescript highlighting.

Co-authored-by: Neil <4138956+nwparker@users.noreply.github.com>
---
 mobile/src/session/mobile-file-syntax.ts      |   4 +-
 .../monarch-embed-entry-recursion.test.ts     |  40 +++++++
 .../monaco-languages/register-glimmer.test.ts |  89 ++++++++++++++
 .../lib/monaco-languages/register-glimmer.ts  | 109 ++++++++++++++++++
 src/renderer/src/lib/monaco-setup.ts          |   2 +
 .../editor-language-detect.test.ts            |   6 +
 .../editor-language/editor-language-detect.ts |   2 +
 7 files changed, 251 insertions(+), 1 deletion(-)
 create mode 100644 src/renderer/src/lib/monaco-languages/register-glimmer.test.ts
 create mode 100644 src/renderer/src/lib/monaco-languages/register-glimmer.ts

diff --git a/mobile/src/session/mobile-file-syntax.ts b/mobile/src/session/mobile-file-syntax.ts
index 9756b3eb6c3..4156291b2f4 100644
--- a/mobile/src/session/mobile-file-syntax.ts
+++ b/mobile/src/session/mobile-file-syntax.ts
@@ -50,7 +50,9 @@ const LANGUAGE_ALIASES: Record<string, string> = {
   zsh: 'bash',
   fish: 'bash',
   yml: 'yaml',
-  'cuda-cpp': 'cpp'
+  'cuda-cpp': 'cpp',
+  'glimmer-js': 'javascript',
+  'glimmer-ts': 'typescript'
 }
 
 export function resolveMobileSyntaxLanguage(filePath: string, preferredLanguage?: string): string {
diff --git a/src/renderer/src/lib/monaco-languages/monarch-embed-entry-recursion.test.ts b/src/renderer/src/lib/monaco-languages/monarch-embed-entry-recursion.test.ts
index f0d05443cb9..2a28ab57c48 100644
--- a/src/renderer/src/lib/monaco-languages/monarch-embed-entry-recursion.test.ts
+++ b/src/renderer/src/lib/monaco-languages/monarch-embed-entry-recursion.test.ts
@@ -11,6 +11,7 @@ import {
   tokenizeLines
 } from './monarch-tokenizer-test-harness'
 import { astroMonarchLanguage } from './register-astro'
+import { glimmerTsMonarchLanguage } from './register-glimmer'
 import { svelteMonarchLanguage } from './register-svelte'
 import { vueMonarchLanguage } from './register-vue'
 
@@ -202,3 +203,42 @@ describe('vue embed-entry recursion', () => {
     expect(measurement.maxNestedDepth).toBeGreaterThan(0)
   })
 })
+
+describe('glimmer embed-entry recursion', () => {
+  const islandLine = (count: number): string =>
+    `const a = 1; ${'<template>a</template>'.repeat(count)}`
+
+  it('stays within the embed budget for a line of template islands', () => {
+    const ramp = [50, 200, 500, 900].filter(
+      (count) => islandLine(count).length < MAX_TOKENIZATION_LINE_LENGTH
+    )
+    expect(ramp.length).toBeGreaterThanOrEqual(3)
+
+    const depths = ramp.map((count) =>
+      measureNestedDepth(createMonarchTokenizer('glimmer-ts', glimmerTsMonarchLanguage), [
+        islandLine(count)
+      ])
+    )
+
+    for (const measurement of depths) {
+      expect(measurement.error).toBeUndefined()
+      expect(measurement.maxNestedDepth).toBeLessThanOrEqual(EMBED_ENTRY_REST_OF_LINE_BUDGET)
+    }
+  })
+
+  it('re-embeds script and template after over-budget lines', () => {
+    const long = 'x'.repeat(EMBED_ENTRY_REST_OF_LINE_BUDGET)
+    const embeds = endEmbeddedLanguages(
+      tokenizeLines(createMonarchTokenizer('glimmer-ts', glimmerTsMonarchLanguage), [
+        `const s = "${long}"`,
+        'const b = 1',
+        `<template>${long}x`,
+        '  {{b}}',
+        '</template>',
+        'const c = 1'
+      ])
+    )
+
+    expect(embeds).toEqual([null, 'typescript', null, 'handlebars', null, 'typescript'])
+  })
+})
diff --git a/src/renderer/src/lib/monaco-languages/register-glimmer.test.ts b/src/renderer/src/lib/monaco-languages/register-glimmer.test.ts
new file mode 100644
index 00000000000..cd3db94134d
--- /dev/null
+++ b/src/renderer/src/lib/monaco-languages/register-glimmer.test.ts
@@ -0,0 +1,89 @@
+import { describe, expect, it, vi } from 'vitest'
+import { formatTokenizedLines, tokenizeMonarchDocument } from './monarch-tokenizer-test-harness'
+import {
+  glimmerJsMonarchLanguage,
+  glimmerLanguageConfiguration,
+  glimmerTsMonarchLanguage,
+  registerGlimmerLanguages
+} from './register-glimmer'
+
+describe('registerGlimmerLanguages', () => {
+  // Tokenizing cannot observe registration IDs/extensions/idempotence; structural checks cover that contract.
+  it('registers both Glimmer languages, tokenizers, and configuration once', () => {
+    const languages: { id: string }[] = [{ id: 'typescript' }]
+    const register = vi.fn((entry: { id: string }) => {
+      languages.push({ id: entry.id })
+    })
+    const setMonarchTokensProvider = vi.fn()
+    const setLanguageConfiguration = vi.fn()
+    const monacoMock = {
+      languages: {
+        register,
+        setMonarchTokensProvider,
+        setLanguageConfiguration,
+        getLanguages: () => languages
+      }
+    }
+
+    registerGlimmerLanguages(monacoMock)
+    registerGlimmerLanguages(monacoMock)
+
+    expect(register.mock.calls).toEqual([
+      [{ id: 'glimmer-ts', extensions: ['.gts'], aliases: ['Glimmer TS'] }],
+      [{ id: 'glimmer-js', extensions: ['.gjs'], aliases: ['Glimmer JS'] }]
+    ])
+    expect(setMonarchTokensProvider.mock.calls).toEqual([
+      ['glimmer-ts', glimmerTsMonarchLanguage],
+      ['glimmer-js', glimmerJsMonarchLanguage]
+    ])
+    expect(setLanguageConfiguration.mock.calls).toEqual([
+      ['glimmer-ts', glimmerLanguageConfiguration],
+      ['glimmer-js', glimmerLanguageConfiguration]
+    ])
+  })
+})
+
+describe('glimmer tokenization', () => {
+  // Real Monarch tokenization: an embed popped without a matching push throws per line and drops highlighting.
+  it('alternates the script embed with Handlebars template islands', () => {
+    const fixture = `import Component from '@glimmer/component'
+export default class Hello extends Component<Signature> {
+  <template>
+    <p>Hello {{this.name}}</p>
+  </template>
+}
+const Inline = <template>{{@a}}</template>; const Empty = <template></template>;`
+
+    expect(
+      formatTokenizedLines(tokenizeMonarchDocument('glimmer-ts', glimmerTsMonarchLanguage, fixture))
+    ).toMatchInlineSnapshot(`
+      [
+        "import Component from '@glimmer/component' | 0:-@typescript | embed=typescript",
+        "export default class Hello extends Component<Signature> { | 0:-@typescript | embed=typescript",
+        "  <template> | 0:-@typescript 2:tag.gts@glimmer-ts | embed=none",
+        "    <p>Hello {{this.name}}</p> | 0:-@handlebars | embed=handlebars",
+        "  </template> | 0:-@handlebars 2:tag.gts@glimmer-ts | embed=none",
+        "} | 0:-@typescript | embed=typescript",
+        "const Inline = <template>{{@a}}</template>; const Empty = <template></template>; | 0:-@typescript 15:tag.gts@glimmer-ts 25:-@handlebars 31:tag.gts@glimmer-ts 42:-@typescript 58:tag.gts@glimmer-ts 79:-@typescript | embed=typescript",
+      ]
+    `)
+  })
+
+  it('starts a template-only component in Handlebars and uses the javascript embed for .gjs', () => {
+    const fixture = `<template>
+  <h1>{{@title}}</h1>
+</template>
+export const x = 1`
+
+    expect(
+      formatTokenizedLines(tokenizeMonarchDocument('glimmer-js', glimmerJsMonarchLanguage, fixture))
+    ).toMatchInlineSnapshot(`
+      [
+        "<template> | 0:tag.gjs@glimmer-js | embed=none",
+        "  <h1>{{@title}}</h1> | 0:-@handlebars | embed=handlebars",
+        "</template> | 0:tag.gjs@glimmer-js | embed=none",
+        "export const x = 1 | 0:-@javascript | embed=javascript",
+      ]
+    `)
+  })
+})
diff --git a/src/renderer/src/lib/monaco-languages/register-glimmer.ts b/src/renderer/src/lib/monaco-languages/register-glimmer.ts
new file mode 100644
index 00000000000..b305dc862a6
--- /dev/null
+++ b/src/renderer/src/lib/monaco-languages/register-glimmer.ts
@@ -0,0 +1,109 @@
+import type * as Monaco from 'monaco-editor'
+import { restOfLineWithinEmbedBudget } from './monarch-embed-entry-budget'
+
+type GlimmerMonaco = {
+  languages: Pick<
+    typeof Monaco.languages,
+    'getLanguages' | 'register' | 'setLanguageConfiguration' | 'setMonarchTokensProvider'
+  >
+}
+type GlimmerBaseLanguage = 'typescript' | 'javascript'
+
+// Script is the background embed and each `<template>` a Handlebars island; returning through root re-pushes the script embed so `@pop` never runs without one. Like Vue/Svelte, a literal `<template>` in a JS string or comment still switches embeds.
+function createGlimmerMonarchLanguage(
+  baseLanguage: GlimmerBaseLanguage
+): Monaco.languages.IMonarchLanguage {
+  return {
+    defaultToken: '',
+    tokenPostfix: baseLanguage === 'typescript' ? '.gts' : '.gjs',
+    tokenizer: {
+      // Zero-width transitions require @rematch or Monarch silently drops nextEmbedded.
+      root: [
+        [
+          restOfLineWithinEmbedBudget,
+          { token: '@rematch', switchTo: '@script', nextEmbedded: baseLanguage }
+        ],
+        [/(?=.)/, { token: '@rematch', switchTo: '@scriptPlain' }]
+      ],
+      script: [
+        [/<template\s*>/, { token: 'tag', switchTo: '@templateEnter', nextEmbedded: '@pop' }]
+      ],
+      // Over-budget lines keep island boundaries without embeds to bound recursion; re-embed once the rest fits.
+      scriptPlain: [
+        [/<template\s*>/, { token: 'tag', switchTo: '@templateEnter' }],
+        [
+          restOfLineWithinEmbedBudget,
+          { token: '@rematch', switchTo: '@script', nextEmbedded: baseLanguage }
+        ],
+        [/[^<]+/, ''],
+        [/./, '']
+      ],
+      templateEnter: [
+        [
+          restOfLineWithinEmbedBudget,
+          { token: '@rematch', switchTo: '@templateBody', nextEmbedded: 'handlebars' }
+        ],
+        [/(?=.)/, { token: '@rematch', switchTo: '@templatePlain' }]
+      ],
+      templateBody: [
+        [/<\/template\s*>/, { token: 'tag', switchTo: '@root', nextEmbedded: '@pop' }]
+      ],
+      templatePlain: [
+        [/<\/template\s*>/, { token: 'tag', switchTo: '@root' }],
+        [
+          restOfLineWithinEmbedBudget,
+          { token: '@rematch', switchTo: '@templateBody', nextEmbedded: 'handlebars' }
+        ],
+        [/[^<]+/, ''],
+        [/./, '']
+      ]
+    }
+  }
+}
+
+export const glimmerTsMonarchLanguage = createGlimmerMonarchLanguage('typescript')
+export const glimmerJsMonarchLanguage = createGlimmerMonarchLanguage('javascript')
+
+export const glimmerLanguageConfiguration: Monaco.languages.LanguageConfiguration = {
+  comments: { lineComment: '//', blockComment: ['/*', '*/'] },
+  brackets: [
+    ['{', '}'],
+    ['[', ']'],
+    ['(', ')'],
+    ['<', '>']
+  ],
+  autoClosingPairs: [
+    { open: '{', close: '}' },
+    { open: '[', close: ']' },
+    { open: '(', close: ')' },
+    { open: '"', close: '"' },
+    { open: "'", close: "'" },
+    { open: '`', close: '`' }
+  ],
+  surroundingPairs: [
+    { open: '{', close: '}' },
+    { open: '[', close: ']' },
+    { open: '(', close: ')' },
+    { open: '"', close: '"' },
+    { open: "'", close: "'" },
+    { open: '`', close: '`' },
+    { open: '<', close: '>' }
+  ]
+}
+
+const GLIMMER_LANGUAGES = [
+  { id: 'glimmer-ts', extension: '.gts', alias: 'Glimmer TS', language: glimmerTsMonarchLanguage },
+  { id: 'glimmer-js', extension: '.gjs', alias: 'Glimmer JS', language: glimmerJsMonarchLanguage }
+]
+
+export function registerGlimmerLanguages(monaco: GlimmerMonaco): void {
+  const registeredIds = new Set(monaco.languages.getLanguages().map((language) => language.id))
+  for (const { id, extension, alias, language } of GLIMMER_LANGUAGES) {
+    if (registeredIds.has(id)) {
+      continue
+    }
+    monaco.languages.register({ id, extensions: [extension], aliases: [alias] })
+    monaco.languages.setMonarchTokensProvider(id, language)
+    monaco.languages.setLanguageConfiguration(id, glimmerLanguageConfiguration)
+  }
+}
diff --git a/src/renderer/src/lib/monaco-setup.ts b/src/renderer/src/lib/monaco-setup.ts
index 3c7ab05de9d..03c1d2a2dfd 100644
--- a/src/renderer/src/lib/monaco-setup.ts
+++ b/src/renderer/src/lib/monaco-setup.ts
@@ -10,6 +10,7 @@ import htmlWorker from 'monaco-editor/esm/vs/language/html/html.worker?worker'
 import tsWorker from 'monaco-editor/esm/vs/language/typescript/ts.worker?worker'
 import { registerAstroLanguage } from './monaco-languages/register-astro'
 import { registerCudaLanguage } from './monaco-languages/register-cuda'
+import { registerGlimmerLanguages } from './monaco-languages/register-glimmer'
 import { registerJsonlLanguage } from './monaco-languages/register-jsonl'
 import { registerLeanLanguage } from './monaco-languages/register-lean'
 import { registerMarkdownLanguage } from './monaco-languages/register-markdown'
@@ -72,6 +73,7 @@ runMonacoSetupSteps([
   ['Vue language registration', () => registerVueLanguage(monaco)],
   ['Svelte language registration', () => registerSvelteLanguage(monaco)],
   ['Astro language registration', () => registerAstroLanguage(monaco)],
+  ['Glimmer language registration', () => registerGlimmerLanguages(monaco)],
   ['CUDA language registration', () => registerCudaLanguage(monaco)],
   ['Nim language registration', () => registerNimLanguage(monaco)],
   ['Lean language registration', () => registerLeanLanguage(monaco)],
diff --git a/src/shared/editor-language/editor-language-detect.test.ts b/src/shared/editor-language/editor-language-detect.test.ts
index e20c545390a..da44f30501c 100644
--- a/src/shared/editor-language/editor-language-detect.test.ts
+++ b/src/shared/editor-language/editor-language-detect.test.ts
@@ -14,6 +14,12 @@ describe('detectLanguage', () => {
     expect(detectLanguage('src/routes/index.astro')).toBe('astro')
   })
 
+  it('maps Glimmer files to distinct js/ts language ids', () => {
+    expect(detectLanguage('app/components/hello.gjs')).toBe('glimmer-js')
+    expect(detectLanguage('app/components/hello.gts')).toBe('glimmer-ts')
+    expect(detectLanguage('C:\\app\\components\\HELLO.GTS')).toBe('glimmer-ts')
+  })
+
   it('maps Nim files to the nim language id', () => {
     expect(detectLanguage('src/main.nim')).toBe('nim')
     expect(detectLanguage('tasks/build.nims')).toBe('nim')
diff --git a/src/shared/editor-language/editor-language-detect.ts b/src/shared/editor-language/editor-language-detect.ts
index eec2f3b9167..77838f5e447 100644
--- a/src/shared/editor-language/editor-language-detect.ts
+++ b/src/shared/editor-language/editor-language-detect.ts
@@ -103,6 +103,8 @@ const EXT_TO_LANGUAGE: Record<string, string> = {
   '.vue': 'vue',
   '.svelte': 'svelte',
   '.astro': 'astro',
+  '.gjs': 'glimmer-js',
+  '.gts': 'glimmer-ts',
   '.sv': 'systemverilog',
   '.svh': 'systemverilog',
   '.v': 'verilog',