Textual ITerm2 Scrollbar Bug: Double Width

It's a common experience for developers to encounter unexpected visual glitches when working with different terminal emulators. One such peculiar issue has surfaced for users of Textual and iTerm2, specifically concerning the scrollbar's appearance. If you've noticed your scrollbar appearing double its intended width and looking a bit, well, wonky on iTerm2, you're not alone. This article delves into this specific bug, exploring its manifestation and providing a clear, reproducible example.

Understanding the Scrollbar Glitch

Let's get straight to the heart of the matter: the double width scrollbar bug on iTerm2 when using the Textual framework. When you're building applications with Textual, you often rely on its rich set of widgets and its ability to render them beautifully in your terminal. However, the terminal emulator you choose can sometimes introduce unique behaviors. In this case, iTerm2, a popular choice for macOS users, seems to be rendering Textual's scrollbars with an unintended extra width. This doesn't just affect the visual aesthetics; it can also slightly alter the layout and interaction experience within your Textual application. The provided screenshot clearly illustrates this anomaly, showing a scrollbar that occupies more horizontal space than it should, potentially overlapping with other UI elements or simply looking disproportionate.

This issue highlights the intricate dependency between application frameworks like Textual and the underlying terminal environment. While Textual aims for a consistent rendering experience across various terminals, discrepancies can arise due to differences in how terminals interpret and display characters, spacing, and styling. The double width scrollbar suggests a misinterpretation of character widths or padding settings within iTerm2's rendering engine when it communicates with Textual. It's a subtle bug, but for those who appreciate pixel-perfect UIs, it can be quite noticeable and distracting. The fact that it's specific to iTerm2 means that users of other terminals might not experience this problem, further emphasizing the emulator's role in this particular behavior. The goal of this article is to shed light on this specific problem, providing a clear path for anyone encountering it to understand and potentially contribute to a solution.

Reproducing the Double Width Scrollbar

To effectively address and debug an issue, having a reliable method to reproduce it is paramount. Fortunately, the problem of the double width scrollbar on iTerm2 with Textual is straightforward to replicate. The team has provided a concise and elegant code snippet that demonstrates the bug in action. This minimal example serves as a perfect starting point for anyone wanting to verify the issue or contribute to its resolution. Simply running this Python script within your iTerm2 terminal should immediately reveal the exaggerated scrollbar.

from textual.app import App, ComposeResult
from textual.widgets import Static

class ExampleApp(App):
    def compose(self) -> ComposeResult:
        for i in range(250):
            yield Static(f"Hello {i}")

if __name__ == "__main__":
    app = ExampleApp()
    app.run()

This code creates a very basic Textual application. It initializes an App and within its compose method, it yields 250 Static widgets. Each Static widget simply displays a line of text, such as "Hello 0", "Hello 1", and so on. The large number of widgets (250) is crucial because it ensures that the content exceeds the visible height of the terminal window, thereby triggering the display of a scrollbar. When you run this script in iTerm2, the application will render, and you'll observe the scrollbar on the right side of the application window. If you are experiencing the bug, this scrollbar will appear thicker than expected, taking up more horizontal space. This simple yet effective reproduction case is invaluable for developers who are investigating the root cause of the scrollbar's distorted appearance. It isolates the problem to the interaction between Textual's rendering of scrollbars and iTerm2's interpretation of those rendering instructions, stripping away any unnecessary complexity from the application logic itself. By focusing on this minimal reproducible example, the community can more efficiently pinpoint where the rendering discrepancy originates.

Diagnostic Insights: Textual and iTerm2

To provide a clearer picture of the environment where this double width scrollbar bug on iTerm2 occurs, a diagnostic report has been generated using textual diagnose. This report is invaluable as it captures the specific versions of Textual, Rich, Python, the operating system, and crucially, the terminal application being used. Understanding these versions helps in pinpointing potential compatibility issues or regressions that might have been introduced in a specific update.

The provided diagnostics show that Textual version 6.6.0 and Rich version 14.2.0 are in use. These are relatively recent versions, suggesting that the issue might not be due to outdated libraries. The Python version is 3.13.5, running on macOS (Darwin Kernel Version 24.6.0). The most telling piece of information, however, is under the 'Terminal' section. It explicitly states that the 'Terminal Application' is iTerm.app (3.6.5), with TERM set to xterm-256color and COLORTERM set to truecolor. The xterm-256color setting is a common emulation mode, but it's possible that iTerm2's specific implementation or how it handles certain escape codes related to spacing or box-drawing characters differs from what Textual expects, leading to the rendering anomaly. The diagnose output also provides details about the Rich console options, such as the terminal size (width=88, height=25), which can sometimes influence rendering behavior, though in this case, it appears standard. This detailed diagnostic information is the bedrock upon which further investigation will be built, allowing developers to compare environments and identify the exact conditions that trigger the double width scrollbar behavior.

Analyzing the Root Cause

Pinpointing the exact cause of the double width scrollbar on iTerm2 with Textual requires a deeper dive into how terminals render UI elements, especially those involving character widths and spacing. Textual, built on top of the Rich library, relies on terminal capabilities to draw widgets, including scrollbars. Scrollbars are typically rendered using block characters or specific Unicode symbols. The issue likely stems from a discrepancy in how iTerm2 interprets the width of these characters compared to other terminal emulators or the assumptions made by Rich/Textual.

One common source of such problems is the handling of **