Debugger freezes when a breakpoint is hit on a large js file (>256KB) when current scope has local variables when debugging vscode extension webviews

[a1exwang]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior:

2025-12-04.13-12-39.mp4

1. Start from the webview sample in vscode-extension-samples
2. Add a new js file in media/large.js and include it in the HTML.
3. Add a breakpoint in line 3
4. When breakpoint hits, the debugger freezes, unable to resume, stop, etc. I have to reload vscode.
5. (But the Chrome DevTools work well even after vscode debugger freezes)

function getRandomData() {
    const _randomData = "...."; // 257KB long. From my testing, the variable size doesn't matter. What matter is the js file size. This example just use a simple way to make a large js file.)
    console.log("Generating random data..."); // <--------- Add a breakpoint here
    return _randomData[0];
}
setTimeout(_getRandomData, 10000);

Debugging the vscode-js-debugger project I can see the error roots from here:

vscode-js-debug/src/cdp/connection.ts

Line 82 in 68b5caa

const object = JSON.parse(message);

Somehow the input message JSON truncates at 256KB leaving a broken JSON when the script file is larger than 256KB. The broken message is something like:

{"id":1101,"sessionId":"B8FA00E601393C70BB8D058BB70BDD23","result":{
  "result":[{
    "configurable":true,"enumerable":true,"isOwn":true,"name":"_randomData","value": {
      "type":"string","value": "<long content but truncated here

In some other cases, the broken message is a response for getScriptContent request.

Log File
the last exception: (If you need the full log you can ping me internally)

{"tag":"runtime.exception","timestamp":1764825754284,"message":"Unhandled error in debug adapter","metadata":{"message":"Unterminated string in JSON at position 262144 (line 1 column 262145)","stack":"SyntaxError: Unterminated string in JSON at position 262144 (line 1 column 262145)\n\tat JSON.parse (<anonymous>)\n\tat It._onMessage (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:63:7593)\n\tat c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:63:7103\n\tat L.fire (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:44:11104)\n\tat r.<anonymous> (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:108:30452)\n\tat By (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:51607)\n\tat r.n (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:50896)\n\tat r.emit (node:events:519:28)\n\tat qx.zY (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:65730)\n\tat qx.emit (node:events:519:28)\n\tat qx.dataMessage (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:43619)\n\tat qx.getData (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:42862)\n\tat qx.startLoop (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:39432)\n\tat qx._write (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:38716)\n\tat writeOrBuffer (node:internal/streams/writable:572:12)\n\tat _write (node:internal/streams/writable:501:10)\n\tat Writable.write (node:internal/streams/writable:510:10)\n\tat Socket.qy (c:\\Users\\<username>\\.vscode\\extensions\\ms-vscode.js-debug-nightly-2025.11.2517\\src\\extension.js:12:66450)\n\tat Socket.emit (node:events:519:28)\n\tat addChunk (node:internal/streams/readable:561:12)\n\tat readableAddChunkPushByteMode (node:internal/streams/readable:512:3)\n\tat Readable.push (node:internal/streams/readable:392:5)\n\tat TCP.onStreamRead (node:internal/stream_base_commons:189:23)"},"level":3}

Version: 1.106.3 (user setup)
Commit: bf9252a2fb45be6893dd8870c0bf37e2e1766d61
Date: 2025-11-25T22:28:18.024Z
Electron: 37.7.0
ElectronBuildId: 12781156
Chromium: 138.0.7204.251
Node.js: 22.20.0
V8: 13.8.258.32-electron.0
OS: Windows_NT x64 10.0.26100

ms-vscode.js-debug-nightly: 2025.11.2517 (the bug also happens in the stable version)

launch.json:

{
	"version": "0.2.0",
	"configurations": [
		{
			"name": "Run Extension",
			"type": "extensionHost",
			"request": "launch",
			"runtimeExecutable": "${execPath}",
			"debugWebviews": true,
			"autoAttachChildProcesses": true,
			"debugWebWorkerHost": true,
			"args": ["--extensionDevelopmentPath=${workspaceRoot}"],
			"outFiles": ["${workspaceFolder}/out/**/*.js"],
			"preLaunchTask": "npm: watch",
			"trace": {
				"logFile": "C:/Users/**/debugger.log",
				"stdio": true,
			}
		}
	]
}

Additional context
Add any other context about the problem here.

[a1exwang]

This seems to be already fixed in the latest vscode stable release.