Cursor changes from blinking to steady after some ncurses programs terminate

[profbbrown]

Certain ncurses programs cause the cursor to stop blinking after they terminate. I noticed this with programs like cmatrix and vim. Run either of these programs and the cursor will change from blinking to steady after the program finishes.

The problem is related to the use of curs_set in the program. To test this, I wrote a very simple ncurses program:

#include <stdio.h>
#include <ncurses.h>

int main()
{
    initscr();
    curs_set(0);
    refresh();
    curs_set(1);
    endwin();
}

Compile with gcc foo.c -l ncurses.

Before the program is run, the cursor is blinking. After, it is not.

As a control, I tested the same program in iTerm2. The cursor remains blinking after the program terminates.

We can isolate the problem to the curs_set calls. Change the program above by commenting out the curs_set lines. When the program runs, the cursor behaves as it should: blinking both before and after.

Ghostty version: 1.1.0
OS: MacOS 15.3

Config file:

font-family = SF Mono
font-size = 16
cursor-style=block
window-width=120
window-height=40
shell-integration-features=no-cursor

[00-kat]

By the way, what's your ncurses version? It's possible that this is an ncurses bug that was already fixed (I can't reproduce this on ncurses 6.5, as I had mentioned in the Discord server).

[profbbrown]

By default it's 6.0, the version that comes with MacOS 15.3.

I just installed ncurses 6.5 via brew and compiled it in. No change in behavior for me.

clang -I/opt/homebrew/Cellar/ncurses/6.5/include -L/opt/homebrew/Cellar/ncurses/6.5/lib testncurses.c -l ncurses

[mitchellh]

I haven't fully looked at the CSI sequences being set but I suspect curs_set isn't restoring blink. I can reproduce this with Terminal.app and Kitty as well. I don't think this is a Ghostty issue.

When shell integration is enabled, the cursor style does revert to blinking because the prompt line forces a specific cursor style. But without shell integration enabled, we only have blink on startup due to the cursor-style-blink config. After that, it looks like the style is being overwritten.

Still looking though.

[mitchellh]

Found a detailed comment in our source explaining what is happening:

        // If we are setting cursor blinking, we ignore it if we have
        // a default cursor blink setting set. This is a really weird
        // behavior so this comment will go deep into trying to explain it.
        //
        // There are two ways to set cursor blinks: DECSCUSR (CSI _ q)
        // and DEC mode 12. DECSCUSR is the modern approach and has a
        // way to revert to the "default" (as defined by the terminal)
        // cursor style and blink by doing "CSI 0 q". DEC mode 12 controls
        // blinking and is either on or off and has no way to set a
        // default. DEC mode 12 is also the more antiquated approach.
        //
        // The problem is that if the user specifies a desired default
        // cursor blink with `cursor-style-blink`, the moment a running
        // program uses DEC mode 12, the cursor blink can never be reset
        // to the default without an explicit DECSCUSR. But if a program
        // is using mode 12, it is by definition not using DECSCUSR.
        // This makes for somewhat annoying interactions where a poorly
        // (or legacy) behaved program will stop blinking, and it simply
        // never restarts.
        //
        // To get around this, we have a special case where if the user
        // specifies some explicit default cursor blink desire, we ignore
        // DEC mode 12. We allow DECSCUSR to still set the cursor blink
        // because programs using DECSCUSR usually are well behaved and
        // reset the cursor blink to the default when they exit.
        //
        // To be extra safe, users can also add a manual `CSI 0 q` to
        // their shell config when they render prompts to ensure the
        // cursor is exactly as they request.

What is happening is this ncurses program is using the ancient DEC mode 12. This disables cursor blink and thereafter never reenables it, as can be seen in the sequences below:

info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 1049 } h }
info(stream): action: terminal.Parser.Action{ .esc_dispatch = ESC ( B }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [  {  } m }                                                                                          info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [  { 4 } l }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 7 } h }                                                                                        info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 25 } l }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [  {  } H }                                                                                          info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [  { 2 } J }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 12 } l }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 25 } h }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [  { 23, 1 } H }
info(stream): action: terminal.Parser.Action{ .csi_dispatch = ESC [ ? { 1049 } l }

Blink is only disabled (mode 12). It is never re-enabled.