Severe output delay (~3.5s) when using useConptyDll: true with PowerShell 7

[lchpersonal]

Description

When using the bundled conpty.dll (useConptyDll: true), PowerShell 7 initialization output is
severely delayed compared to using the system ConPTY. The prompt appears ~3.5 seconds later, making
the terminal feel unresponsive.

Environment

• OS: Windows 11 Pro 10.0.22621
• node-pty version: @homebridge/node-pty-prebuilt-multiarch@0.13.1
• Shell: PowerShell 7.5.4 (pwsh.exe)
• Node.js version: v22.21.1
• Electron version: 28.0.0

Steps to Reproduce

1. Create a PTY process with the following options:
   const pty = require('node-pty');

const ptyProcess = pty.spawn('pwsh.exe', [], {
name: 'xterm-256color',
cols: 80,
rows: 30,
cwd: process.cwd(),
env: process.env,
useConpty: true,
useConptyDll: true // Using bundled conpty.dll
});

let firstDataTime = null;
let promptTime = null;

ptyProcess.onData((data) => {
if (!firstDataTime) {
firstDataTime = Date.now();
console.log('First data:', data.substring(0, 100));
}

if (data.includes('PS ') && !promptTime) {
  promptTime = Date.now();
  console.log('Prompt received after:', promptTime - firstDataTime, 'ms');
}

});

2. Compare with system ConPTY by setting useConptyDll: false

Expected Behavior

PowerShell prompt should appear within ~300-500ms, similar to system ConPTY behavior.

Actual Behavior

With useConptyDll: true, the PowerShell prompt appears after ~3800ms, making the terminal feel very
slow to start.

Detailed Comparison

System ConPTY (useConptyDll: false)

First data received: 83 bytes
Content: [?25l[2J[HPowerShell 7.5.4\r\n]0;C:\Program Files\PowerShell\7\pwsh.exe[?25h
Prompt detected after: 340ms

Bundled ConPTY (useConptyDll: true)

First data received: 4 bytes
Content: [1t
Prompt detected after: 3815ms

Analysis

The key difference is:

1. System ConPTY: First data batch contains PowerShell version info and initialization output (83
   bytes), prompt arrives in 340ms
2. Bundled ConPTY: First data batch contains only an ANSI escape sequence [1t (4 bytes), then waits
   ~3.8 seconds before sending the actual PowerShell output

This suggests that the bundled conpty.dll has a different output buffering or synchronization
mechanism that causes severe delays in forwarding the shell's initialization output.

Impact

This makes the bundled ConPTY unusable for applications that need responsive terminal startup. We
initially wanted to use useConptyDll: true to fix an IME (Input Method Editor) candidate box
flickering issue, but the 3.8-second delay is unacceptable for user experience.

Questions

1. Is this a known issue with the bundled conpty.dll?
2. Are there any configuration options to control the output buffering behavior?
3. What version of ConPTY is bundled, and could updating it help?
4. Is there a way to get the IME stability benefits of bundled ConPTY without the output delay?

Additional Context

• The bundled conpty.dll is located at:
  node_modules/@homebridge/node-pty-prebuilt-multiarch/build/Release/conpty/conpty.dll
• File size: 109,104 bytes
• File date: Feb 28, 2025
• We've also tried the warmupConPtyDll() approach (spawning a dummy PTY at startup), but it doesn't
  help with the per-terminal delay

Workaround

Currently using system ConPTY (useConptyDll: false) for fast startup, but this causes IME issues on
Windows (candidate box flickering during terminal output).