Module semantics 

Layer 2: the semantics seam — a pull-based accessibility tree pass.

Each widget optionally contributes an accesskit::Node describing its role, label, and state (Widget::semantics, defaulted to a no-op so no existing widget is affected). The collection pass mirrors paint, not the arena tree-walk: a container’s children are ChildPods outside the arena, so — exactly like ChildPod::paint_child threads an absolute origin down — ChildPod::semantics_child translates the current absolute origin into the child’s space and recurses.

RenderRoot::semantics drives it after layout (so bounds are valid) and returns a SemanticsUpdate: a flat (NodeId, Node) list plus the root id and the focused node id, ready for a platform adapter (accesskit_*) to consume. This crate owns no platform wiring or per-frame scheduling — those live in each platform shell.

Contributing a node

A leaf widget calls SemanticsCtx::push_node with its role and a closure that sets label/state; bounds are filled from the current absolute origin/size automatically. A widget that also has semantics-visible children (e.g. a scroll view) uses SemanticsCtx::push_container, whose visit closure recurses into the children (via semantics_child) so they become the node’s accesskit children. A transparent container (Flex/Stack/Padding/…) contributes no node of its own — it just calls semantics_child for each child, so their nodes attach to whatever encloses the container.

Structs

SemanticsCtx

Collects accesskit::Nodes during a semantics pass, tracking the current absolute origin/size (mirroring paint’s origin threading) and the parent/child structure.

SemanticsUpdate

A collected accessibility tree produced by RenderRoot::semantics.

Constants

ROOT_NODE_ID

The reserved NodeId of the synthetic window/root node (RenderRoot::semantics always roots the tree here). It is a fixed constant — the one id never drawn from the per-ChildPod allocator — so a platform adapter can treat “the root” as a stable anchor across every frame. Pod-derived ids start well above it (see SemanticsCtx::alloc_base).