Fix text selection for single-frame (short) layouts

[peterkrueck]

Problem

Text selection fails for StructuredText content that is short enough to lay out within a single SwiftUI frame (a reply/fragment that fits without scrolling). On macOS a trackpad drag never starts a selection; the model is never populated. Taller content that settles its layout across multiple frames works fine — so the symptom presents as "selection works on long text, fails on short text."

Root cause

TextSelectionInteraction populates the TextSelectionModel from the Text.LayoutKey preference via:

.overlayTextLayoutCollection { layoutCollection in
  Color.clear
    .onChange(of: AnyTextLayoutCollection(layoutCollection), initial: true) {
      model.setCoordinator(coordinator)
      model.setLayoutCollection(layoutCollection)
    }
}

For short content, all of the progressive Text.LayoutKey preference updates land in one SwiftUI frame. SwiftUI drops an onChange action's repeated same-frame invocations (this is the source of the runtime warning onChange(of:) action tried to update multiple times per frame), so the populated layout collection's onChange action is never delivered. The model is left on an empty layout collection, closestPosition(to:) returns nil, and NSTextInteractionView.mouseDown therefore sets no dragStart — selection never begins. Taller content spreads its layout across frames, so a later (non-coalesced) onChange delivers the populated collection and selection works.

This was confirmed by instrumenting NSTextInteractionView: on a failing short bubble the overlay frame is correct and mouseDown fires (so geometry/hit-testing are fine), but model.hasText == false and closestPosition == nil at click time, long after layout settled. Deferring the model update inside the onChange (e.g. via DispatchQueue.main.async) does not help, because the dropped action never runs to schedule the deferred work.

Fix

Deliver the layout collection to the model through a non-interactive representable's update path instead of onChange:

• A tiny NSViewRepresentable / UIViewRepresentable (TextLayoutModelUpdater) calls model.setCoordinator(_:) + model.setLayoutCollection(_:) from updateNSView / updateUIView.
• update*View runs on every SwiftUI update cycle with the latest inputs and is not subject to the onChange same-frame drop, so the final populated collection always reaches the model.
• The hosted platform view is hit-test transparent (hitTest returns nil / isUserInteractionEnabled = false), so the real selection overlay and any excluded scroll regions (code blocks) keep receiving their events.
• The model's layoutCollection and coordinator are @ObservationIgnored, so writing them from update*View does not trigger SwiftUI invalidation.

This also eliminates the onChange(of:) action tried to update multiple times per frame warning, since the onChange is gone.

Verified on macOS (AppKit) on real hardware: short replies that previously could not be selected now select on the first drag; taller replies are unaffected. The change is platform-symmetric for the UIKit side.

🤖 Generated with Claude Code