Feature: FloatingPanelAdaptiveLayoutAnchor initializer with absolute inset + edge (fixed position, adaptive size)

[fawzirifai]

Summary

Add a new initializer to FloatingPanelAdaptiveLayoutAnchor that pins the panel position using absolute inset + edge (same semantics as FloatingPanelLayoutAnchor(absoluteInset:edge:referenceGuide:)), while keeping adaptive sizing: the panel should still shrink to content when content is small and expand (capped by safe area / contentBoundingGuide) when content is large.

Motivation

• FloatingPanelLayoutAnchor(absoluteInset:edge:referenceGuide:) gives fixed positioning (e.g. “panel top at 8pt from safe area top”) but the panel size is fixed (e.g. full height), not content-driven.
• FloatingPanelAdaptiveLayoutAnchor(absoluteOffset:contentLayout:referenceGuide:contentBoundingGuide:) gives adaptive sizing (shrink to content, cap by safe area) but the position is derived from the content dimension (offset from reference), not from a fixed inset + edge.

There is no way to get both:

1. Position: Pin the panel from a specific edge with a fixed inset (e.g. “top of panel at safe area top + 8pt”).
2. Size: Adaptive to content (shrink when content is small, grow up to safe area when content is large).

That combination is useful for panels that should always start at a consistent place (e.g. below a toast) but still size to their content.

Proposed API

Add an initializer on FloatingPanelAdaptiveLayoutAnchor:

/// Position is pinned from the given edge at a fixed inset (like FloatingPanelLayoutAnchor).
/// Size remains adaptive: panel shrinks to content and is capped by contentBoundingGuide.
init(
    absoluteInset: CGFloat,
    edge: FloatingPanelReferenceEdge,
    contentLayout: UILayoutGuide,
    referenceGuide: FloatingPanelLayoutReferenceGuide,
    contentBoundingGuide: FloatingPanelLayoutContentBoundingGuide = .none
)

Behavior:

• Position: Same as FloatingPanelLayoutAnchor(absoluteInset:absoluteInset, edge: edge, referenceGuide: referenceGuide) — the panel’s leading edge (in the panel’s main axis) is placed at absoluteInset from the given edge of the reference guide (e.g. for a bottom panel with edge: .top, the panel’s top is at referenceGuide.topAnchor + absoluteInset).
• Size: Unchanged from current adaptive behavior — panel height/width follows contentLayout and is capped by contentBoundingGuide (e.g. .safeArea).

Example use case

A panel that should sit below a toast (fixed top inset) but still size to its content:

// Panel position: top pinned at safeArea.top + topAnchorInset
// Panel size: height = content, capped by safe area
.full: FloatingPanelAdaptiveLayoutAnchor(
    absoluteInset: Self.topAnchorInset,  // e.g. 8pt or below toast
    edge: .top,
    contentLayout: targetGuide,
    referenceGuide: .safeArea,
    contentBoundingGuide: .safeArea
)

Thank you for considering this feature.

[scenee]

Thank you for your feedback! We’ll take it into consideration.