Kitty Terminal Bug: Cursor Jumps On Ctrl+L

Have you ever encountered a peculiar glitch in your terminal that makes you scratch your head? Well, users of the kitty terminal emulator have recently stumbled upon just such an issue, specifically with the clear_terminal reset active command. This bug, which appeared in version 0.44.0, causes the cursor to behave unexpectedly after a Ctrl+L command, leading to a few unexpected side effects. Let's dive into what's happening, how to reproduce it, and why it's causing a headache for some users, especially those who rely on custom key bindings.

The clear_terminal reset active Conundrum

When you use clear_terminal reset active in kitty, the intention is to clear the terminal screen and reset its state. However, since version 0.44.0, this command, when combined with a specific key binding for Ctrl+L, introduces a bug where the cursor unexpectedly jumps down an extra line after the initial screen clear. This might seem like a minor visual anomaly, but it can have cascading effects on how other commands and key bindings function within your terminal session. The bug is particularly noticeable when you press Enter a few times after executing the Ctrl+L command. Instead of the cursor moving down a single line with each Enter press, it unexpectedly skips down two lines on the first Enter press. This deviation from the expected single-line movement disrupts the normal flow of terminal interaction and can lead to unexpected behavior, especially in scripts or workflows that rely on precise cursor positioning.

Reproducing the Glitch

To witness this peculiar behavior yourself, you'll need to follow a specific set of steps. First, launch kitty with a configuration that disables shell integration and sets up a custom key binding for Ctrl+L. The command to do this is:

kitty --config NONE -o 'shell_integration disabled' -o 'map ctrl+l combine : clear_terminal reset active : signal_child SIGWINCH'

This command ensures that kitty starts with a clean slate, no shell integration interfering, and your Ctrl+L mapped to the problematic clear_terminal reset active action along with a SIGWINCH signal. Once kitty is running, you need to execute any command that will fill up your terminal screen a bit, so there's something to clear. After that, simply press Ctrl+L. You should see the screen clear. Now, the crucial part: press Enter a few times. Observe the cursor's movement. On the first press of Enter, you'll notice the cursor moves down two lines instead of the usual single line. Subsequent Enter presses will behave normally, moving the cursor down one line at a time. This distinct jump on the first Enter press is the core of the observed bug.

Visualizing the Difference:

To truly understand the impact, comparing the behavior with previous versions is essential. Screenshots provided by the reporter clearly show this discrepancy. In version 0.44.0, the cursor's double jump is evident. However, when looking at version 0.43.1, the cursor behaves as expected, moving down precisely one line with each Enter press after the Ctrl+L. This visual evidence strongly suggests that the change in cursor movement was introduced specifically in version 0.44.0.

The Impact Beyond Cursor Movement

While the cursor jumping down two lines might be a minor annoyance for some, it has more significant implications for users who have customized their terminal experience with specific key bindings. The reporter, for instance, intentionally disables shell integration to use a custom Ctrl+L behavior that forces their Bash prompt to redraw. This is a common practice for users who want a cleaner prompt or need to ensure certain shell features are always available. The bug introduced in kitty 0.44.0 breaks a specific Bash binding: bind -x "\C-x\C-b": _fzf_git_branches. This binding is used to trigger a function that interacts with Git branches, likely using a tool like fzf for interactive selection. The problem arises because if you press Ctrl+L and don't press Enter immediately afterward, the subsequent key combination Ctrl+X, Ctrl+B is not interpreted correctly by Bash. Instead of triggering the _fzf_git_branches function, the literal characters ^X^B appear in the terminal. This effectively disables the custom key binding until you press Enter at least once after clearing the screen.

This means that users who rely on this type of workflow are now forced to add an extra step—pressing Enter—after clearing their screen just to ensure their custom key bindings function correctly. This is not just an inconvenience; it's a disruption of established workflows and can lead to errors or unexpected behavior in automated scripts or interactive sessions. The need to press Enter after Ctrl+L to make other key bindings work properly highlights how deeply intertwined these terminal behaviors can be and how a seemingly small graphical glitch can have functional consequences.

Environment Details

To help diagnose and fix this issue, it's crucial to understand the environment in which it occurs. The bug was reported on a system running kitty 0.44.0 (commit hash efbfcf7923). The underlying operating system is Ubuntu 20.04.6 LTS, running on a x86_64 architecture with kernel version 5.15.0-161-generic. The terminal is being run under X11 with OpenGL 4.6 (Core Profile Mesa 21.2.6). The font configuration includes DejaVu Sans Mono for regular, bold, italic, and bold-italic styles.

The kitty executable is located at /home/precor/.local/kitty.app/bin/kitty, and its base directory is /home/precor/.local/kitty.app/lib/kitty. The system shell is /bin/bash. Importantly, the configuration overrides include shell_integration disabled and the specific key binding map ctrl+l combine : clear_terminal reset active : signal_child SIGWINCH. This setup is key to reproducing the bug, as it disables the default shell integration and forces the custom Ctrl+L behavior.

Several environment variables were observed by the kitty process, including PATH, LANG set to en_US.UTF-8, SHELL as /bin/bash, DISPLAY as :0, and various XDG variables indicating a GNOME desktop session on Ubuntu. The system color scheme is set to no_preference with no theme applied.

This detailed environment information is vital for developers to replicate the issue accurately and identify potential conflicts or dependencies that might be contributing to the bug. Understanding the specific version of kitty, the operating system, the display server (X11), and the exact configuration used provides a clear path for debugging.

Why This Matters: Beyond a Simple Visual Glitch

It's easy to dismiss the cursor jumping down an extra line as a minor visual quirk. However, in the world of terminal usage, precision and predictability are paramount. For users who have invested time in customizing their terminal environment, especially those who disable shell integration to gain finer control over their prompt and key bindings, this bug represents a significant disruption. The fact that it breaks a seemingly unrelated Bash binding (bind -x "\C-x\C-b": _fzf_git_branches) by preventing key combinations from being passed to the shell until an intermediate Enter press is a clear indicator of a deeper issue than just cursor positioning.

This bug highlights the intricate relationship between the terminal emulator, the shell, and custom configurations. When kitty introduces a change in how it handles screen clearing and cursor movement, it can inadvertently affect how the shell interprets subsequent input. The signal_child SIGWINCH part of the key binding is likely intended to notify the child process (the shell) about a window size change, which often happens after a clear. However, the extra cursor movement seems to interfere with the input buffer or signal handling of the shell, preventing key sequences from being processed correctly until the buffer is