Struct LayoutCtx 

#[non_exhaustive]
pub struct LayoutCtx<'a> {
    pub container: Rect,
    pub children: &'a [El],
    pub measure: &'a dyn Fn(&El) -> (f32, f32),
    pub rect_of_key: &'a dyn Fn(&str) -> Option<Rect>,
    pub rect_of_id: &'a dyn Fn(&str) -> Option<Rect>,
}

Context handed to a LayoutFn. Marked #[non_exhaustive] so future fields (intrinsic-at-width, scroll context, …) can be added without breaking author code that currently reads container / children / measure.

Fields (Non-exhaustive)

Non-exhaustive structs could have additional fields added in future. Therefore, non-exhaustive structs cannot be constructed in external crates using the traditional Struct { .. } syntax; cannot be matched against without a wildcard ..; and struct update syntax will not work.

container: Rect

Inner rect of the parent (after padding) — the area available for child placement. Children may be positioned anywhere; the library does not clamp returned rects to this region.

children: &'a [El]

Direct children of the node, in source order. Read-only — return positions through your Vec<Rect>.

measure: &'a dyn Fn(&El) -> (f32, f32)

Intrinsic (width, height) for any child. Wrapped text returns its unwrapped width here; if you need width-dependent wrapping you’ll need to size the child with Fixed / Fill instead.

rect_of_key: &'a dyn Fn(&str) -> Option<Rect>

Look up any keyed node’s laid-out rect. Returns None when the key is absent from the tree, when the node hasn’t been laid out yet (siblings later in source order), or when the key was used on a node without a recorded rect. Used by widgets like crate::widgets::popover::popover to position children relative to elements outside their own subtree.

rect_of_id: &'a dyn Fn(&str) -> Option<Rect>

Look up a node’s laid-out rect by its computed_id. Same semantics as Self::rect_of_key but skips the key → computed_id translation — useful for runtime-synthesized layers (tooltips, focus rings) that anchor to a node the library already knows by id.