CollapsingHeader

use zaxis::CollapsingHeader;
let out = CollapsingHeader::new("appearance", "Appearance")
    .default_open(true)
    .show_with_actions(ui, |ui| ui.button("Reset").clicked(), |ui| {
        ui.checkbox(&mut animations, "Animations");
    });
if out.actions { animations = true; }

new(id_source, caption) requires identity separate from the caption. Repeated captions and reordered siblings retain independent state. Ui::collapsing is the simple closed-by-default shortcut. id_source replaces the constructor key. default_open applies only at initialization; open(&mut bool) is authoritative every pass and receives user changes once. Output contains header_response, logical open, changed, body: Option<R> and the actions closure's result.

The complete header including chevron is one activation region. A pointer release inside or focused Enter/Space release toggles it once. Right actions have their own stable scope, are measured once, aligned right and clipped at narrow widths. Their input does not toggle the disclosure. Use explicit Ids for dynamic action controls. As with other deferred layouts, use Ui::displayed_rect/Context::visual_rect for screen coordinates of responses returned by actions. Popup actions inherit local theme and use the existing Popup layers.

enabled(false) disables header/actions/content. expandable(false) locks the current expansion while leaving actions and content usable. icon(ImageSource) uses Image's existing resource pipeline. No icon pack is required.

Height animation uses Ui::reveal_with: the body runs at most once per pass, measures actual current height, and retargets from displayed height during reversal. Initially closed content is not built. During exit it may be built for painting, but loses input immediately. Focus in the closing subtree returns to the header. Once closed it allocates no height, builds no body and requests no animation frames. Clips, scroll routing and neighboring layout use displayed height. Nested retained headers survive hiding by their ancestor; omitting the whole ancestor releases them. The application must continue to evaluate the component to observe events.

animate_height(false) gives immediate allocation. Chevron rotation shares the motion engine and is independent of height animation. Reduced motion snaps both. CollapsingStyle exposes shared DisclosureStyle header geometry and ControlStyle state surfaces, content padding, spacing, animate_height and motion. These optional fields merge with inherited Style::collapsing, including explicit zero/transparent values. Zero header height intentionally hides the header. Theme changes retain state.

Run cargo run --example collapsing_headers. --smoke-test verifies native presentation only. The example includes nesting, right actions, repeated captions, reordering, controlled expansion, icons and dark/light/high-contrast themes.

AccessibilityCopy Anchor Link

The header is a Button named by its caption with an expanded state. It accepts Click, Expand and Collapse; a header that is not expandable accepts none of them. Widgets in the header row follow it in the tree as separate nodes. See Accessibility.