PowerShell sample code that demonstrates a PSReadLine key handler that display meta-information about the command being typed in real time.

PSReadLineMetaInfoKeyHandler.ps1

<#

 License: MIT
 Author:  Michael Klement <[email protected]>

 See https://stackoverflow.com/a/67266971/45375 for more information.

#>

# The printable characters to respond to.
$printableChars = [char[]] (0x20..0x7e + 0xa0..0xff)
# The control characters to respond to.
$controlChars = 'Enter', 'Escape', 'Backspace', 'Delete'

# Set up the key handler for all specified characters.
$printableChars + $controlChars | ForEach-Object {

  Set-PSReadLineKeyHandler $_ { 
    param($key, $arg)

    $line = $cursor = $null
    [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref] $null, [ref] $cursor)

    # Handle the key at hand.
    switch ($key.Key) {
      'Backspace' { [Microsoft.PowerShell.PSConsoleReadLine]::BackwardDeleteChar(); break }
      'Delete' { try { [Microsoft.PowerShell.PSConsoleReadLine]::Delete($cursor, 1) } catch { }; break } # ignore error with empty buffer 
      'Escape' { 
        [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref] $line, [ref] $null)
        [Microsoft.PowerShell.PSConsoleReadLine]::Delete($0, $line.Length)
        break 
      }
      'Enter' {
        # Clear any previous meta-information output, so that it doesn't linger and get mixed with command output.
        try {
          # !! On conhost.exe (regular console) windows on Windows, [Console]::CursorTop and [Console]::WindowTop are *relative to the scrollback buffer*.
          # !! In Windows Terminal and on Unix, [Console]::WindowTop is always 0, and [Console]::CursorTop is relative to the screen height - even in the presence of a scrollback buffer.
          Write-Host -NoNewLine (, (' ' * [Console]::WindowWidth) * ([Console]::WindowTop + [Console]::WindowHeight - [Console]::CursorTop - 1) -join "`n")
        }
        catch { Write-Warning "`nClearing the screen below the current line failed: $_" } # This shouldn't happen.

        # !! Workaround for a display bug: If the cursor isn't at the very end of the line, everything to the
        # !! right is inexplicably *erased* on submission, even though the submission itself still works fine.
        # !! We detect that case and simply fill the entire buffer again, which leaves it drawn correctly on submission.
        # !! (Note that [Microsoft.PowerShell.PSConsoleReadLine]::SetCursorPosition($line.Length) does *not* work.)
        [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref] $line, [ref] $cursor)
        if ($cursor -ne $line.length) {
          [Microsoft.PowerShell.PSConsoleReadLine]::Delete(0, $line.Length)
          [Microsoft.PowerShell.PSConsoleReadLine]::Insert($line)
        }

        # Submit the command.
        [Microsoft.PowerShell.PSConsoleReadLine]::AcceptLine()
        return # We're done.
      }
      Default { [Microsoft.PowerShell.PSConsoleReadLine]::Insert($key.KeyChar) }
    }

    # Get the updated buffer content and cursor position.
    [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref]$line, [ref] $cursor)
    # Note: To get the *AST* (too), use the following:
    #    $ast = $tokens = $errors = $cursor = $null
    #    [Microsoft.PowerShell.PSConsoleReadLine]::GetBufferState([ref] $ast, [ref] $tokens, [ref] $errors, [ref] $cursor)
    
    # Determine the meta-informaton to print:
    $metaInfo = $key | Out-String

    # !! There are TWO PROBLEMS with printing meta-information below the line being edited:
    # !! * ON *PASTING* TEXT ONTO THE COMMAND LINE:
    # !!    * With *true pasting*, available on Windows only, with Ctrl-V, this handler isn't invoked at all.
    # !!      This isn't much of a problem, as just typing a space afterwards, for instance, will resume display of the meta-information.
    # !!    * !! With *simulated typing* - right-click on Windows, all forms of pasting on Unix - the meta-information is printed 
    # !!      !! *many times, stacked*, as the keypresses are being simulated, and the cursor line *doesn't show the command being entered*.
    # !!      !! It is unclear why PSReadLine seems to lose the position of its input line and for now WE CANNOT SOLVE THIS PROBLEM.
    # !!  * IF THERE AREN'T ENOUGH LINES IN THE WINDOW BELOW THE LINE BEING EDITED TO FIT THE META-INFORMATION,
    # !!    we need to scroll up to make enough room. Otherwise every keystroke causes the cursor to stay on the last line,
    # !!    and the meta-information to print *above* it, with each keystroke's meta-info pushing the previous output upward.
    # !!    The only terminal that does *not* require this and *automatically* scrolls up is conhost.exe (regular console windows) on Windows.
    # !!    ?? Is this connected to all other terminals not being scrollback-buffer-aware (see above)?
    # !!    WE CANNOT FULLY FIX THIS PROBLEM, AS WE WOULD HAVE TO TELL PSReadLine TO GET ITS INPUT FROM A DIFFERENT LINE,
    # !!    NAMELY THE SCROLLED-UPWARD ONE; THE BEST WE CAN DO IS TO ANTICIPATE THE PROBLEM AND CLEAR THE SCREEN
    # !!    using [Microsoft.PowerShell.PSConsoleReadLine]::ClearScreen() - which is NON-DESTRUCTIVE: the scrollback
    # !!    buffer isn't cleared, and even the current screen content is merely scrolled out of view.
    # !!    By using a PSReadLine method to clear the screen, PSReadLine implicitly knows what line to prompt for input on afterwards, i.e. the first.
    if ($env:OS -ne 'Windows_NT' -or $env:WT_SESSION) {
      # Workaround for all terminals except conhost.exe
      if ([Console]::CursorTop + $metaInfo.Count -gt [Console]::WindowTop + [Console]::WindowHeight) {
        [Microsoft.PowerShell.PSConsoleReadLine]::ClearScreen()
      }
    }
    
    # Print the desired information below the line being edited.
    # Note:
    #   * The .PadRight() calls ensure that all lines are fully filled (padded with spaces),
    #     in order to erase potential remnants from previously displayed information.
    #   * This is NOT sufficient to deal with *varying line counts* being displayed, however.
    Write-Host # blank line
    Write-Host -NoNewLine -ForegroundColor Yellow ($metaInfo -split '\r?\n' | ForEach-Object PadRight ([Console]::WindowWidth-1), ' ')

    # Set the new cursor position.
    [Microsoft.PowerShell.PSConsoleReadLine]::SetCursorPosition($cursor)
  }
}