Scroll area

use ui::scroll;

div().relative()                                // the bar is absolute in here
    .child(
        div()
            .id("pane")
            .size_full()
            .overflow_y_scroll()
            .track_scroll(&self.scroll)         // gpui's handle, the app's field
            .child(content),
    )
    .child(scroll::scrollbar("pane-bar", &self.scroll, &self.scroll_bar))

scroll::thumb(viewport, max_offset, offset, scroll::MIN_THUMB) // -> Option<Range<Pixels>>
scroll::offset_for_thumb(top, viewport, max_offset, size)      // the inverse

// ui::scroll

/// An overlay, not a gutter, so one arriving never reflows the content. Shows
/// nothing when the content fits, and takes no `&Theme` — the thumb is `ink`.
pub fn scrollbar(
    id: impl Into<SharedString>,
    handle: &ScrollHandle,
    state: &ScrollbarState,
) -> gpui::AnyElement;

/// A field on your view, holding where in the thumb a drag was grabbed —
/// without it the thumb jumps its middle to the pointer on every press.
pub struct ScrollbarState;

/// `None` when there is nothing to scroll, when the viewport is zero, or when
/// the thumb would be shorter than `MIN_THUMB`.
pub fn thumb(
    viewport: Pixels,
    max_offset: Pixels,
    offset: Pixels,
    min: Pixels,
) -> Option<Range<Pixels>>;

pub fn offset_for_thumb(top: Pixels, viewport: Pixels, max_offset: Pixels, size: Pixels) -> Pixels;

// ...