Fission 0.6.3: ScrollIntoView runtime effects

That is the missing piece for document editors, page outlines, validation summaries, search results, sidebars, and any app where a user action selects something that may already exist lower in a scrollable surface.

Why this belongs in the runtime

Fission already had scroll primitives:

•

Scroll owns a scrollable viewport;

•

RuntimeState.scroll stores offsets;

•

pointer wheels, scrollbars, and text-input caret tracking can mutate those offsets internally;

•

app code can read previous layout through View::get_rect(...).

The missing feature was a safe way for application intent to say: after the next layout pass, scroll this viewport so that widget is visible.

Reducers cannot safely mutate scroll offsets because they do not own layout geometry. Widget conversion must stay side-effect free. ScrollIntoViewRequest keeps that boundary intact.

The API

A reducer can now emit:

ctx.effects.scroll_into_view(ScrollIntoViewRequest {
    container: Some(WidgetId::explicit("document.canvas.scroll")),
    target: WidgetId::explicit("document.page.3"),
    axis: ScrollAxis::Vertical,
    alignment: ScrollAlignment::Start,
    padding: [24.0, 24.0, 24.0, 24.0],
    behavior: ScrollBehavior::Instant,
    if_needed: false,
});

The request is resolved after layout, when Fission can see both the target rect and the scroll container geometry. If container is omitted, the runtime uses the nearest matching scroll ancestor.

ScrollAlignment supports Start, Center, End, Nearest, and Fraction(f32). Nearest keeps already visible content stationary and otherwise uses the smallest offset change needed to reveal the target.

Document editor flow

A document app can keep pages in natural order:

Scroll {
    id: Some(WidgetId::explicit("document.canvas.scroll")),
    direction: FlexDirection::Column,
    flex_grow: 1.0,
    child: Some(DocumentPages { pages: state.pages.clone() }.into()),
    ..Default::default()
}
.into()

Each page wrapper gets a stable explicit ID such as document.page.3. When the user selects page 3 from a page rail or outline, the reducer updates selected state and emits ScrollIntoViewRequest. Fission handles the offset after layout and schedules another frame so rendering, hit testing, semantics, and test-control tree output converge on the new scroll position.

That avoids the wrong workaround: reordering rendered pages just to make the selected page appear near the top.

Runtime behavior

The runtime now:

queues ScrollIntoViewRequest as a runtime effect;

waits for the next layout snapshot;

resolves the target and container;

computes the requested offset in scroll-content coordinates;

clamps the offset to the scrollable range;

writes RuntimeState.scroll;

schedules a follow-up frame when the offset changes.

Missing targets are retried once on the next frame. That covers common route or conditional-render cases where the state change that selects a target also causes it to appear.

Migration notes

Applications using 0.6.2 can update the Rust dependency in the usual way:

fission = { version = "0.6.3", default-features = false, features = ["desktop"] }

Use stable explicit widget IDs for both the scroll container and reveal targets when the request comes from app state, route state, search state, or external document navigation.

Verification

0.6.3 includes focused runtime tests for explicit containers, nearest scroll ancestors, alignment, visibility no-ops, queued runtime effects, and one-frame retry behavior. The release also runs formatting, workspace checks, and documentation-site checks before publishing the crate set.

Older post

Fission 0.6.2: accessibility, text input, and editor-grade focus

Newer post

Fission 0.7.0: streaming capabilities and deterministic async boundaries

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Fission

A cross-platform, GPU-accelerated user interface framework for Rust. MIT licensed.

Ready to use today. Widget APIs are expected to remain stable; some runtime and shell APIs may change before 1.0.0.

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Reference

Fission 0.7.0