# llimphi-widget-scroll

Reusable vertical scroll area.

Until now every app rebuilt scrolling by hand: `App::on_wheel` + an offset in the
`Model` + `clip` + virtualization. This widget packages that pattern into a single
builder, **with no state of its own** (the offset still lives in the `Model`,
true to the Elm loop):

- a **clipped viewport** of fixed height (`viewport_len`),
- **content displaced** by `-offset` px (overflow clipped),
- a **draggable scrollbar** on the right (only when the content exceeds the
  viewport),
- a **self-contained wheel** through `View::on_scroll`: turning the wheel with
  the cursor over the area emits a `Msg` without the app routing anything through
  its global `on_wheel`.

The caller must know the content's **total height** (`content_len`) and the
**visible height** (`viewport_len`) — just as `list`/`grid` already ask for the
visible window. For uniform rows that is `n_rows * row_height`.

## The `on_scroll` callback convention

`on_scroll` receives the **delta in px** to add to the offset (not the absolute
offset): both the wheel and the scrollbar drag emit deltas, and the caller
accumulates and clamps in its `update` with `clamp_offset`. It is the same idea
as the `splitter` (the drag handler is reused for the whole drag, so a captured
absolute offset would go stale; a delta per event is always correct).

```ignore
// view:
scroll_y(
    model.offset,
    model.rows.len() as f32 * ROW_H,
    panel_h,
    list_view,
    Msg::ScrollBy,            // Fn(f32) -> Msg, arg = delta px
    &ScrollPalette::default(),
)
// update:
Msg::ScrollBy(d) => {
    m.offset = clamp_offset(m.offset + d, content_len, viewport_len);
}
```

To bring a selection into view (keyboard) see `ensure_visible`; for smooth or
inertial scrolling see `approach`.

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Part of **llimphi** — see [llimphi](../../README.md).