Unsuccessful proof of concept for fixing Unix Console.ReadKey without native binaries.

PlatformUnix.cs

using System;
using System.Runtime.InteropServices;

namespace Microsoft.PowerShell.Internal
{
    internal class PlatformUnix
    {
        // termios
        private const int STDIN_FILENO = 0; // File descriptor for Standard Input.
        private const int NCCS = 64; // Size of the array c_cc for control characters.

        // Control Characters (c_cc)
        private const uint VTIME = 5; // Timeout in deciseconds.
        private const uint VMIN = 6; // Minimum number of characters to expect.

        // Local Modes (c_lflag)
        private const uint ICANON = 0000002; // Canonical input (erase and kill processing).
        private const uint ECHO = 0000010; // Enable echo.
        private const uint IEXTEN = 0100000; // Enable extended input character processing.

        // Input Modes (c_iflag)
        private const uint IXON = 0002000; // Enable start/stop output control.
        private const uint IXOFF = 0010000; // Enable start/stop input control.

        // Attribute Selection (tcsetattr)
        private const int TCSANOW = 0; // Change attributes immediately.

        // Poll Events
        private const ushort POLLIN = 0x0001; // Standard Input is ready.

        internal static ConsoleKeyInfo ReadKey()
        {
            BlockUntilKeyAvailable();
            return Console.ReadKey(intercept: true);
        }

        [DllImport("libc")]
        private static extern uint tcgetattr(int fd, IntPtr termios_p);

        [DllImport("libc")]
        private static extern uint tcsetattr(int fd, int optional_actions, IntPtr termios_p);

        [DllImport("libc")]
        private static extern uint poll(ref pollfd fds, uint nfds, int timeout);

        private static void BlockUntilKeyAvailable()
        {
            pollfd fds;
            fds.fd = STDIN_FILENO;
            fds.events = POLLIN;
            fds.revents = 0;

            termios newTerminal = GetTerminal();
            termios previousTerminal = newTerminal;
            newTerminal.c_iflag &= ~(IXON | IXOFF);
            newTerminal.c_lflag &= ~(ECHO | ICANON | IEXTEN);
            newTerminal.c_cc[VMIN] = 1;
            newTerminal.c_cc[VTIME] = 0;
            SetTerminal(newTerminal);
            try
            {
                poll(ref fds, 1, -1);
            }
            finally
            {
                SetTerminal(previousTerminal);
            }
        }

        private static termios GetTerminal()
        {
            IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(termios)));
            try
            {
                if (tcgetattr(STDIN_FILENO, ptr) >= 0)
                {
                    return Marshal.PtrToStructure<termios>(ptr);
                }

                return default(termios);
            }
            finally
            {
                Marshal.FreeHGlobal(ptr);
            }
        }

        private static bool SetTerminal(termios termios)
        {
            IntPtr ptr = Marshal.AllocHGlobal(Marshal.SizeOf(typeof(termios)));
            try
            {
                Marshal.StructureToPtr<termios>(termios, ptr, fDeleteOld: false);
                if (tcsetattr(STDIN_FILENO, TCSANOW, ptr) <= 0)
                {
                    return true;
                }

                return false;
            }
            finally
            {
                Marshal.FreeHGlobal(ptr);
            }
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct termios
        {
            // Input Mode Flags
            internal uint c_iflag;

            // Output Mode Flags
            internal uint c_oflag;

            // Control Mode Flags
            internal uint c_cflag;

            // Local Mode Flags
            internal uint c_lflag;

            internal byte c_line;

            /// Control Characters
            [MarshalAs(UnmanagedType.ByValArray, SizeConst=NCCS)]
            internal byte[] c_cc;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct pollfd
        {
            internal uint fd;

            internal ushort events;

            internal ushort revents;
        }
    }
}