366. A kept tab is hidden, not removed

Date: 2026-09-17

Status

Accepted. Closes book/src/TODO.md’s “A tab’s content is rebuilt when it is selected again”. Adds Styled.isHidden() to :core.

Context

§5 asks tabs for “lazy content instantiation”, and a strip builds only the selected tab’s content. The TODO entry said that is right, and that it means a scroll position, a caret or a half-typed form in a background tab is gone, with nowhere to put it but the application’s model.

Keeping content alive needs a subtree that stays mounted, with its elements and their state, while it takes no layout, paint, input or focus. Widget.nothing() removes a subtree’s content, and :disabled keeps a subtree painted. The toolkit had nothing in between.

Decision

Styled.isHidden() keeps a node and its subtree mounted and unused. tabs with keep-alive wraps each tab it has shown in a keyed tab-page that is hidden unless selected.

• Styled.isHidden() defaults to false. WidgetRenderer.render returns no box for a hidden node and does not render under it, so it has no layout, no paint, no hit-test region, and no Measured or Located notifications. PointerRouter.isFocusable refuses anything under a hidden ancestor, walking up as isDisabled does, and refocus lets go of a focus that has become hidden.
• Tabs.keepAlive(boolean) and keep-alive=#true in markup. TabsState keeps the values it has shown and still has, in first-shown order. The panel then holds one TabPage(value, content, selected) per kept tab, keyed by value so the reconciler matches a page to the same elements whichever tab is selected.
• A tab never selected is still not built. A closed tab is dropped from the kept set, and its page unmounts.
• Off by default, which is §5’s default.

Consequences

• Hidden content still costs its elements and their state, and a rebuild of a hidden subtree still runs when its widgets change; only rendering is skipped.
• isHidden is not a stylesheet feature, so §8’s subset still has no display: none.
• The showcase’s chapter strip keeps its tabs alive, and each chapter has an unbound note field to show it; gallery-navigation is re-blessed.