Tabs (selector with SelectorStyle::Tabs)

Note (migration): tabs are now selector(sel).style(SelectorStyle::Tabs), a one-of-N selector bound to a Signal<String> of the active tab key (docs/navigation.md). The prose below describes the tab semantics and native mapping, which are unchanged; the standalone tabs() builder was folded into selector.

A tabs() host is a native tabbed container: several keyed destinations, one visible at a time, switched by a native tab widget. It reuses the same route registry as nav() (docs/navigation.md), so a tab key is a route: you select tabs, deep-link to them, and drive/assert them from dayscript the same way you navigate.

tabs()
    .tab("overview", tr("overview"), overview_page)
    .tab("details",  tr("details"),  details_page)
    .tab("settings", tr("settings"), settings_page)
    .id("main-tabs")

navigate("settings") selects the settings tab; a deep link to settings lands on it; dayscript navigate {route: settings} / assert_route {route: settings} drive and check it.

Semantics

• Keyed destinations: each .tab(key, title, build) is addressed by key. title is the tab label; build runs once at mount.
• All pages resident: every tab’s content is built eagerly and kept alive, so each tab preserves its own state across switches, which is how every native tab container behaves.
• .selected(key) picks the initial tab (default: the first). Startup deep links still win.
• Nesting & fall-through: Hosts register on a stack (docs/navigation.md). tabs() inside a nav() route registers on top: navigate("<tab-key>") selects the tab, while navigate("<some-nav-route>") (a key the tabs host doesn’t know) falls through to the enclosing nav(), which replaces the page (disposing the tabs host, whose scope cleanup unregisters its controller). current_route() reports the innermost host: the active tab.

The wire (spec)

day_spec:

• kinds::TABS (host) and kinds::TABS_PAGE (one tab’s content container; its frame is native-owned, like a nav page).
• props::TabsProps { titles: Vec<String>, selected: usize }; TabsPatch::Selected(usize) (programmatic sync, applied without echoing a SelectionChanged back, per the from-native rule).
• props::TabsPageProps { title }: the page’s tab label, read by the host on insert.

The framework side (day-pieces) registers a NavController whose push selects a tab by key, current reports the active tab key, and pop is a no-op (tabs have no back-stack). Native tab selection arrives as Event::SelectionChanged; the host lays out each page’s content at the size the tab widget reports via Event::FrameChanged.

Native mapping

The widget owns page content on every backend (the user-visible choice is a native tab widget per platform; GTK, having adopted libadwaita, uses the Adwaita segmented switcher (a .linked toggle group) over an AdwViewStack, since Adwaita has no icon-free tab widget):

Each page reports its allocated content size (FrameChanged) so Day lays out the tab’s content at native size, the same mechanism nav pages use. Pages with native-owned frames are skipped by set_frame.

Deep links & dayscript

Because tab keys are routes, everything that already targets routes works unchanged:

• Deep link: launching with the deep link settings selects the settings tab once the tabs host is mounted (warm links arrive as Custom("deeplink") and re-navigate).

• dayscript:

  - navigate: { route: tabs }        # enter the tabs route (nav pushes it; tabs registers)
  - assert_route: { route: overview } # innermost host = the active tab
  - navigate: { route: details }      # select a tab by key
  - assert_route: { route: details }
  - assert_value: { id: main-tabs, value: 1 } # the tabs host records the active index

Testing

• e2e (day-mock, crates/day-pieces/tests/mock_e2e.rs): eager page build, select-by-key + index recording, native-selection → current_route, no-redundant-patch idempotence, and the nested-in-nav fall-through (leaving a tab route disposes the tabs host). Plus a deep-link test.
• Showcase + walkthrough: the tabs route hosts three keyed tabs; the walkthrough enters it, selects each tab by key, asserts the route/index, and screenshots, verified on all five local targets (macos-appkit/gtk/qt, ios-uikit, android-mdc).