Silent PDF Printing Fails With Multiple Instances

Environment

Version: 0.23.2 SDK and DIST
Arch: x64, though this should also affect x86 builds
OS: Confirmed on Linux and Windows 10

Brief

When a new window is spawned via new_instance:true, only one unattended (i.e., no user interaction) PDF print job can take place at a time.

If more than one "print-to-pdf" job is running concurrently (in separate renderers), the pdf_path option passed to nw.Window.print is seemingly ignored and a "Save As" dialog is spawned by Chromium's printing plugin.

Use case

The use case here is silently generating large batches of PDFs in parallel.

Demo

capture

Steps to Reproduce

package.json

{
  "name": "nwjs-issue-5968",
  "main": "index.js"
}

index.js

'use strict';

/*
  Bug: Any (i > 1) will cause subsequent instances to prompt the user for a
       file name, even though one is specified and passed to `nw.Window.print`
       in the newly spawned instance. Setting `new_instance:false` "solves" the
       problem, but the jobs are now processed serially.
*/

for(let i = 0; i < 3; ++i) {
  nw.Window.open('print.html', {
    show: true,
    new_instance: true
  });
}

print.html

<!doctype html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Test Page</title>
    <script>
      document.addEventListener('DOMContentLoaded', () => {
        // Random ID for PDF file name and payload.
        const docId = Math.random().toString(36).slice(2);

        document.body.innerText = docId;

        const win = nw.Window.get();

        // Print to PDF. Should be able to complete without user interaction.
        win.print({
          pdf_path: `./${docId}.pdf`
        });

        setTimeout(() => win.close(), 1000);
      });
    </script>
  </head>
  <body>
  </body>
</html>

nwjs / nw.js

Silent PDF Printing Fails With Multiple Instances #5968