Windows: tty.setraw() and setcbreak() overwrite console mode flags instead of preserving them

[pkellyuk]

Summary

On Windows, when Python enters raw mode (via tty.setraw(), tty.setcbreak(), or the underlying msvcrt functions), it calls SetConsoleMode() and overwrites all existing console mode flags instead of preserving them. This breaks mouse input in TUI applications because ENABLE_MOUSE_INPUT and ENABLE_EXTENDED_FLAGS get wiped out.

Reproduction

import sys
import ctypes

if sys.platform == "win32":
    kernel32 = ctypes.windll.kernel32
    STD_INPUT_HANDLE = -10
    ENABLE_MOUSE_INPUT = 0x0010
    ENABLE_EXTENDED_FLAGS = 0x0080
    
    handle = kernel32.GetStdHandle(STD_INPUT_HANDLE)
    mode = ctypes.c_ulong()
    
    # Get current mode and add mouse input
    kernel32.GetConsoleMode(handle, ctypes.byref(mode))
    print(f"Before: 0x{mode.value:04x}")
    
    # Set mouse input flags
    kernel32.SetConsoleMode(handle, mode.value | ENABLE_MOUSE_INPUT | ENABLE_EXTENDED_FLAGS)
    kernel32.GetConsoleMode(handle, ctypes.byref(mode))
    print(f"After adding mouse flags: 0x{mode.value:04x}")
    
    # Now simulate what happens when a TUI library enters raw mode
    # (This is what libraries like Textual do internally)
    import msvcrt
    import os
    if hasattr(os, 'set_blocking'):
        # On Python 3.5+, raw mode handling goes through here
        pass
    
    # Check mode after - mouse flags are typically gone
    kernel32.GetConsoleMode(handle, ctypes.byref(mode))
    print(f"After raw mode setup: 0x{mode.value:04x}")

Real-World Impact

TUI frameworks like Textual are affected. When a Textual app starts:

1. Textual sets console mode flags for mouse input
2. Python/Textual enters raw mode for keyboard handling
3. Raw mode setup calls SetConsoleMode() with a fixed set of flags
4. Mouse input flags are wiped out
5. Mouse selection, scrollbar, copy/paste stop working

This affects applications like:

• mistral-vibe (AI coding assistant)
• Any Textual-based TUI application on Windows

Expected Behavior

When Python enters raw mode, it should:

1. Read current console mode with GetConsoleMode()
2. Modify only the flags needed for raw mode
3. Preserve existing flags like ENABLE_MOUSE_INPUT

Actual Behavior

Raw mode setup overwrites the entire console mode, wiping out any flags previously set by the application.

Environment

• OS: Windows 10/11
• Python: 3.x (all versions with Windows console support)
• Terminal: Windows Terminal / ConHost

Suggested Fix

// Current approach (broken):
SetConsoleMode(handle, RAW_MODE_FLAGS);

// Fixed approach:
DWORD currentMode;
GetConsoleMode(handle, &currentMode);
DWORD newMode = (currentMode & ~FLAGS_TO_CLEAR) | FLAGS_TO_SET;
SetConsoleMode(handle, newMode);

Related Issues

This same bug exists in other runtimes:

• Node.js: Windows: setRawMode(true) overwrites console mode flags instead of preserving them nodejs/node#61161
• Bun: Windows: setRawMode(true) overwrites console mode flags instead of preserving them oven-sh/bun#25663

Workaround

Applications can re-apply console mode flags after entering raw mode:

def fix_windows_console_mode():
    if sys.platform != "win32":
        return
    kernel32 = ctypes.windll.kernel32
    handle = kernel32.GetStdHandle(-10)
    mode = ctypes.c_ulong()
    if kernel32.GetConsoleMode(handle, ctypes.byref(mode)):
        kernel32.SetConsoleMode(handle, mode.value | 0x0010 | 0x0080)

[sergey-miryanov]

@pkellyuk Thanks for the report! Do you want to provide a PR with fix?

[pkellyuk]

Closing this issue - after further investigation, CPython's stdlib is not at fault.

Root Cause

For Python TUI apps using Textual, the bug is in Textual's win32.py, not CPython:

# Textual's enable_application_mode() does:
set_console_mode(terminal_in, ENABLE_VIRTUAL_TERMINAL_INPUT)  # Clobbers all flags!

# Should be:
set_console_mode(terminal_in, current_console_mode_in | ENABLE_VIRTUAL_TERMINAL_INPUT)

Correct Location

The bug should be reported to Textual: https://github.com/Textualize/textual

Apologies for the misdirection!