Background colour sometimes ignored in Windows Terminal

[clinton-r]

Environment

Microsoft Windows [Version 10.0.19041.746]
Windows Terminal Version: 1.4.3243.0
Microsoft Visual Studio Community 2019 Version 16.8.4

Steps to reproduce

Compile the test program below.
Open Windows Terminal cmd or Powershell tab. Run the program. (Might have to run it several times to see the problem.)

#include <stdio.h>
#include <stdlib.h>
#include <windows.h>

static HANDLE hStdin = NULL;
static HANDLE hStdout = NULL;

typedef struct
{
    int fgCode;
    int bgCode;
    char* colourName;

} VT_COLOUR_INFO;

void writeString(char* str)
{
#if 1
    WriteConsoleA(hStdout, str, strlen(str), NULL, NULL);
#else
    printf(str);
#endif
}

int main(void)
{
    char buf[1000];
    DWORD mode;

    // Get handles for stdin and stdout
    hStdin = GetStdHandle(STD_INPUT_HANDLE);
    if (hStdin == INVALID_HANDLE_VALUE || hStdin == NULL)
        return 1;
    hStdout = GetStdHandle(STD_OUTPUT_HANDLE);
    if (hStdout == INVALID_HANDLE_VALUE || hStdout == NULL)
        return 1;

    // Enable escape sequences
    if (!GetConsoleMode(hStdout, &mode))
        return 1;
    if (!SetConsoleMode(hStdout, mode | ENABLE_VIRTUAL_TERMINAL_PROCESSING))
        return 1;

    if (!GetConsoleMode(hStdin, &mode))
        return 1;
    if (!SetConsoleMode(hStdin, mode | ENABLE_VIRTUAL_TERMINAL_INPUT))
        return 1;

    // TEST OUTPUT

    // SGR colour codes
    VT_COLOUR_INFO colourLookup[] =
    {
        { 30, 40, "black" },
        { 31, 41, "red" },
        { 32, 42, "green" },
        { 33, 43, "yellow" },
        { 34, 44, "blue" },
        { 35, 45, "magenta" },
        { 36, 46, "cyan" },
        { 37, 47, "white" },

        { 90, 100, "bright black" },
        { 91, 101, "bright red" },
        { 92, 102, "bright green" },
        { 93, 103, "bright yellow" },
        { 94, 104, "bright blue" },
        { 95, 105, "bright magenta" },
        { 96, 106, "bright cyan" },
        { 97, 107, "bright white" },
    };

    writeString("normal text\r\n");

    for (int bg = 0; bg < (sizeof(colourLookup) / sizeof(colourLookup[0])); bg += 1)
    {
        sprintf_s(buf, sizeof(buf), "\x1b[%dm", colourLookup[bg].bgCode); // set background colour
        writeString(buf);
        for (int fg = 0; fg < (sizeof(colourLookup) / sizeof(colourLookup[0])); fg += 1)
        {
            writeString("\x1b[2K"); // erase line (set line to background colour)
            sprintf_s(buf, sizeof(buf), "\x1b[%dm", colourLookup[fg].fgCode); // set foreground colour
            writeString(buf);
            sprintf_s(buf, sizeof(buf), "%16s on %16s ",
                colourLookup[fg].colourName, colourLookup[bg].colourName);
            writeString(buf);
            writeString("\x1b[7mnegative ");    // SGR negative on
            writeString("\x1b[27m");            // SGR negative off
            writeString("\x1b[4munderline ");   // SGR underline on
            writeString("\x1b[24m\r\n");        // SGR underline off
        }
    }

    writeString("\x1b[0m"); // SGR defaults
    writeString("\x1b[2K"); // erase line
    writeString("\r\nnormal text\r\n");

    if (!SetConsoleMode(hStdin, mode))
        return 1;

    return 0;
}

Expected behavior

The test program uses SGR VT sequences to show all the different foreground and background colours, as well as negative and underlined text.

Actual behavior

For each line, the program first erases the line before printing text on it, to set the entire line to the background colour. It appears that sometimes black is used instead of the background colour. Also, spaces are sometimes printed as black instead of the background colour as well. Here is an example with both line and a space:

Notes

Doesn't happen in conhost.

Something similar happens when the terminal scrolls down - usually the new blank line gets the background colour as it should, but occasionally it gets black instead. (Try commenting out the first line in the for loop.)

Thanks

[zadjii-msft]

Weird. I don't think this is #3848, though it does seem similar.

[j4james]

This looks to me like a problem in the conpty space optimization - on some lines it's just not sending through the ESC [ K to clear the line. It's sensitive to timing, so by inserting sleeps in different parts of the test code, you can affect how often the problem shows up (or even prevent it showing up).

Issue #5430 might be related.

[zadjii-msft]

We believe this was fixed in Terminal v1.13, in #11982. @clinton-r would you mind confirming?

[clinton-r]

We believe this was fixed in Terminal v1.13, in #11982. @clinton-r would you mind confirming?

Hi @zadjii-msft , I just tried it again with version 1.13.10395.0 and the problem is gone. Thanks!

I had also mentioned this in the Notes section:

Something similar happens when the terminal scrolls down - usually the new blank line gets the background colour as it should, but occasionally it gets black instead. (Try commenting out the first line in the for loop.)

(Note I meant the first line of the inner for loop.)

That is still the case, but I think it makes sense for it to be behaving like this - I assume it colours the full line when scrolling down, before I set the new colours and write text. Here's what it looks like:

[zadjii-msft]

Yep, that's correct. That's more in line with what other Terminal emulators do. Thanks for confirming!