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Author SHA1 Message Date
Finn Eitreim
ce0848af5c
gpui: Fix bottom-aligned scroll bar disappearing (#51223) 
Closes #51198

This actually doesn't effect any of the scrollbars in Zed, as they have
a separate handler that prevents this issue from occurring
in `crates/ui/src/components/scrollbar.rs`, line 856
```rust
let current_offset = current_offset
      .along(axis)
      .clamp(-max_offset, Pixels::ZERO)
      .abs();
```

so it is gpui specific. I still added a test case and I have a manual
test script:
<details><summary>Details</summary>
<p>

```rust
//! Reproduction of the scrollbar-offset bug in bottom-aligned `ListState`.
//!
//! Run with: cargo run -p gpui --example list_bottom_scrollbar_bug
//!
//! The list starts pinned to the bottom. Before the fix, the red scrollbar
//! thumb was pushed off the bottom of the track (invisible).
use gpui::{
    App, Bounds, Context, ListAlignment, ListState, Window, WindowBounds, WindowOptions, div, list,
    prelude::*, px, rgb, size,
};
use gpui_platform::application;

const ITEM_COUNT: usize = 40;
const COLORS: [u32; 4] = [0xE8F0FE, 0xFCE8E6, 0xE6F4EA, 0xFEF7E0];

struct BugRepro {
    list_state: ListState,
}

impl BugRepro {
    fn new() -> Self {
        let list_state = ListState::new(ITEM_COUNT, ListAlignment::Bottom, px(5000.));
        Self { list_state }
    }
}

impl Render for BugRepro {
    fn render(&mut self, _window: &mut Window, _cx: &mut Context<Self>) -> impl IntoElement {
        let state = &self.list_state;

        let max_offset = state.max_offset_for_scrollbar().y;
        let raw_offset = -state.scroll_px_offset_for_scrollbar().y;
        let viewport_h = state.viewport_bounds().size.height;
        let content_h = max_offset + viewport_h;

        let thumb_h = if content_h > px(0.) {
            ((viewport_h / content_h) * viewport_h).max(px(20.))
        } else {
            viewport_h
        };

        let thumb_top = if max_offset > px(0.) {
            (raw_offset / max_offset) * (viewport_h - thumb_h)
        } else {
            px(0.)
        };

        div()
            .size_full()
            .flex()
            .flex_row()
            .bg(rgb(0xffffff))
            .text_color(rgb(0x333333))
            .text_xl()
            .child(
                div().flex_1().h_full().child(
                    list(state.clone(), |ix, _window, _cx| {
                        let height = if ix % 4 == 0 { px(70.) } else { px(40.) };
                        let bg = COLORS[ix % COLORS.len()];
                        div()
                            .h(height)
                            .w_full()
                            .bg(rgb(bg))
                            .border_b_1()
                            .border_color(rgb(0xcccccc))
                            .px_2()
                            .flex()
                            .items_center()
                            .child(format!("Item {ix}"))
                            .into_any()
                    })
                    .h_full()
                    .w_full(),
                ),
            )
            .child(
                div()
                    .w(px(14.))
                    .h_full()
                    .bg(rgb(0xe0e0e0))
                    .relative()
                    .child(
                        div()
                            .absolute()
                            .right(px(0.))
                            .top(thumb_top)
                            .h(thumb_h)
                            .w(px(14.))
                            .rounded_sm()
                            .bg(rgb(0xff3333)),
                    ),
            )
    }
}

fn main() {
    application().run(|cx: &mut App| {
        let bounds = Bounds::centered(None, size(px(400.), px(500.)), cx);
        cx.open_window(
            WindowOptions {
                window_bounds: Some(WindowBounds::Windowed(bounds)),
                ..Default::default()
            },
            |_, cx| cx.new(|_| BugRepro::new()),
        )
        .unwrap();
        cx.activate(true);
    });
}
```

</p>
</details> 

where I was able to test it out, here is a video of the new working
behavior.


https://github.com/user-attachments/assets/02e26308-da18-418b-97fc-dd52a3325dab

Release Notes:

- gpui: fixed a bug where the scollbar would disappear when using a
bottom aligned list.

---------

Co-authored-by: Mikayla Maki <mikayla.c.maki@gmail.com>
 2026-03-25 22:29:40 +00:00