Preflight Checklist

• I have read the Contributing Guidelines for this project.
• I agree to follow the Code of Conduct that this project adheres to.
• I have searched the issue tracker for a bug report that matches the one I want to file, without success.

Electron Version

43.0.0-nightly.20260413

What operating system(s) are you using?

Other Linux

Operating System Version

Reproduces on any host OS when installing an Electron binary for a different target OS via the postinstall script. Demonstrated here on Linux downloading a darwin / win32 build.

What arch are you using?

x64

Last Known Working Electron version

No response

Does the issue also appear in Chromium / Google Chrome?

No

Expected Behavior

ELECTRON_INSTALL_PLATFORM is documented in docs/tutorial/installation.md as the supported way to override the target platform for the Electron postinstall download. Setting it should make the postinstall script:

1. Download the zip for the requested platform.
2. Write path.txt containing the executable path for that platform's build (electron.exe for win32, Electron.app/Contents/MacOS/Electron for darwin/mas, etc.).
3. Produce an isInstalled() result that reflects what is actually on disk, so subsequent installs don't re-download unnecessarily.

Actual Behavior

npm/install.js resolves the target platform in two places and the two disagree when ELECTRON_INSTALL_PLATFORM is used.

The download path, added in #49981, honors the new env var:

// npm/install.js, line ~20
const platform = process.env.ELECTRON_INSTALL_PLATFORM
  || process.env.npm_config_platform
  || process.platform;

But getPlatformPath(), which produces the executable path written to path.txt and compared during isInstalled(), was not updated:

// npm/install.js, line ~98
function getPlatformPath() {
  const platform = process.env.npm_config_platform || os.platform();
  ...
}

Consequences on, for example, a Linux host running ELECTRON_INSTALL_PLATFORM=darwin npm install electron:

• The darwin zip is downloaded and extracted to dist/ (correct).
• path.txt is written as electron (the Linux executable name), not Electron.app/Contents/MacOS/Electron.
• On a subsequent install, isInstalled() runs fs.existsSync(path.join(__dirname, 'dist', 'electron')), which does not exist in a darwin build, so the cache check fails and the zip is re-downloaded and re-extracted every time.
• Any tooling that reads path.txt to locate the Electron executable will read the wrong filename for the artifact that was actually extracted.

Testcase Gist URL

No response

Additional Information

Minimal simulation of the branch taken by getPlatformPath():

// Simulating npm/install.js on a Linux host with ELECTRON_INSTALL_PLATFORM=darwin
process.env.ELECTRON_INSTALL_PLATFORM = 'darwin';

// Download resolution (after #49981): uses 'darwin' — downloads darwin zip
const downloadPlatform = process.env.ELECTRON_INSTALL_PLATFORM
  || process.env.npm_config_platform
  || process.platform;  // 'darwin'

// path.txt resolution (getPlatformPath): ignores ELECTRON_INSTALL_PLATFORM
const pathTxtPlatform = process.env.npm_config_platform || require('os').platform();
// => 'linux' on Linux, so path.txt is written as 'electron' alongside a darwin build.

The fix is a one-liner in getPlatformPath() that brings it in line with the download resolution:

 function getPlatformPath() {
-  const platform = process.env.npm_config_platform || os.platform();
+  const platform = process.env.ELECTRON_INSTALL_PLATFORM || process.env.npm_config_platform || os.platform();

I will open a PR implementing this.