Console.ReadLine() returns each line one Enter late when Console.TreatControlCAsInput is true on Windows #133814

Description

With Console.TreatControlCAsInput = true on Windows, Console.ReadLine() does not return when Enter is pressed. It returns when the next line is entered, and what it returns is the previous line. An app reading lines in that mode is always one line behind the user.

The setter clears ENABLE_PROCESSED_INPUT and nothing else (src/libraries/System.Console/src/System/ConsolePal.Windows.cs:475-482). With that flag clear, the console host delivers Enter as a bare \r: calling ReadConsoleW directly on the input handle, a line comes back ending in \r, where the default mode gives \r\n. Console.ReadLine() calls Console.In.ReadLine(), and Console.In on Windows wraps a StreamReader over the console stream (ConsolePal.Windows.cs:179-189), and StreamReader.ReadLine, having found \r, calls ReadBuffer() to see whether a \n follows (src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs:860-863). On a console that read waits for the next line. The bare \r is measured; that the look-ahead is what waits comes from reading the source. In the same mode Console.Read() hands back the \r without waiting for another line, which points at the look-ahead rather than at the host.

Found while re-testing #44667, which is about the same property.

Reproduction Steps

Console.TreatControlCAsInput = true;

while (true)
{
    string? line = Console.ReadLine();
    Console.WriteLine($"ReadLine returned: \"{line}\"");
}

A net10.0 console app with implicit usings. Run it in a console window, type one and Enter, two and Enter, three and Enter.

Expected behavior

Each ReadLine() returns on its own Enter, as it does with the first line commented out:

Actual behavior

The console screen buffer after three + Enter:

ReadLine returned: "one"
ReadLine returned: "two"

The typed text is not on screen either: the echoed Enter returns the cursor to column 0 of the same row without moving to a new one, so the next output overwrites it.

Regression?

No. The same lag, measured with a probe that reads three lines the same way, on .NET Core 3.1.4 and .NET Framework 4.8.

Known Workarounds

None tested.

Configuration

Windows 11 build 26200, x64, .NET 10.0.12, inbox conhost.exe. Also measured under a standalone OpenConsole.exe from Windows Terminal v1.0.1401.0 (2020), with the same result. Not run on Linux or macOS.

Other information

A related Windows-only quirk of the same reader is #40735 (Console.In.Peek() returning -1 after the first line).

I have not built a fix. Searching issues for ReadLine TreatControlCAsInput, ReadLine ENABLE_PROCESSED_INPUT, Console.ReadLine enter twice and ReadLine previous line console found no earlier report.

I'd like to work on fixing this, including working out where the fix belongs, and would appreciate being assigned.