From d7e8e12f84e8b7eefd38996a663331f9ee9d8792 Mon Sep 17 00:00:00 2001
From: Adam Dangoor <adamdangoor@gmail.com>
Date: Mon, 15 Jun 2026 18:18:43 +0100
Subject: [PATCH] Fix state-during-view-update faults in text selection

With text selection enabled, SwiftUI logged non-fatal faults on layout
passes:

  onChange(of: Layout) action tried to update multiple times per frame.
  onChange(of: AnyTextLayoutCollection) action tried to update multiple times per frame.
  Modifying state during view update, this will cause undefined behavior.

`AppKitTextSelectionView` stored selection rectangles in `@State` and
seeded them from two `onChange(initial: true)` handlers (`selectedRange`
and `layout`), both writing the same state during the initial view
update. Derive the rectangles as an `@Observable`-tracked computed
property instead: no `@State`, no `onChange`, no state mutation during
the update, while still recomputing whenever the selected range or layout
changes.

`TextSelectionModel.setLayoutCollection` re-wrote the observed
`selectedRange` during reconciliation even when the reconciled value was
unchanged; as the overlay's GeometryReader/preferences converge this runs
several times per frame. Only assign when the value actually changes.

Fixes #22.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
---
 .../AppKit/AppKitTextSelectionView.swift      | 26 +++++++++++--------
 .../Shared/TextSelectionModel.swift           | 11 ++++++--
 2 files changed, 24 insertions(+), 13 deletions(-)

diff --git a/Sources/Textual/Internal/TextInteraction/AppKit/AppKitTextSelectionView.swift b/Sources/Textual/Internal/TextInteraction/AppKit/AppKitTextSelectionView.swift
index 2c793a89..0b9adab8 100644
--- a/Sources/Textual/Internal/TextInteraction/AppKit/AppKitTextSelectionView.swift
+++ b/Sources/Textual/Internal/TextInteraction/AppKit/AppKitTextSelectionView.swift
@@ -11,7 +11,6 @@
 
   struct AppKitTextSelectionView: View {
     @Environment(TextSelectionModel.self) private var textSelectionModel: TextSelectionModel?
-    @State private var selectionRects: [TextSelectionRect] = []
 
     private let layout: Text.Layout
     private let origin: CGPoint
@@ -22,7 +21,8 @@
     }
 
     var body: some View {
-      Group {
+      let selectionRects = self.selectionRects
+      return Group {
         if selectionRects.isEmpty {
           Color.clear
         } else {
@@ -37,18 +37,22 @@
           }
         }
       }
-      .onChange(of: textSelectionModel?.selectedRange, initial: true, updateSelectionRects)
-      .onChange(of: layout, initial: true, updateSelectionRects)
     }
 
-    private func updateSelectionRects() {
-      if let textSelectionModel,
-        let selectedRange = textSelectionModel.selectedRange
-      {
-        selectionRects = textSelectionModel.selectionRects(for: selectedRange, layout: layout)
-      } else {
-        selectionRects = []
+    /// Selection rectangles for the current range within this layout.
+    ///
+    /// Derived in `body` rather than stored in `@State` and seeded from an
+    /// `onChange(initial: true)`: that initial action wrote state during the first
+    /// view update, which SwiftUI flags ("Modifying state during view update", and
+    /// previously "tried to update multiple times per frame" when both the
+    /// `selectedRange` and `layout` handlers fired on the same frame). As an
+    /// `@Observable`-tracked computation it still recomputes whenever the selected
+    /// range or `layout` changes, with no state mutation.
+    private var selectionRects: [TextSelectionRect] {
+      guard let textSelectionModel, let selectedRange = textSelectionModel.selectedRange else {
+        return []
       }
+      return textSelectionModel.selectionRects(for: selectedRange, layout: layout)
     }
   }
 #endif
diff --git a/Sources/Textual/Internal/TextInteraction/Shared/TextSelectionModel.swift b/Sources/Textual/Internal/TextInteraction/Shared/TextSelectionModel.swift
index b7baae00..8881231d 100644
--- a/Sources/Textual/Internal/TextInteraction/Shared/TextSelectionModel.swift
+++ b/Sources/Textual/Internal/TextInteraction/Shared/TextSelectionModel.swift
@@ -60,11 +60,18 @@
         return
       }
 
-      // Try to reconcile the selected text range
-      self.selectedRange = layoutCollection.reconcileRange(
+      // Try to reconcile the selected text range. Only write when it actually
+      // changes: as the overlay's GeometryReader/preferences converge, this runs
+      // several times per frame with the same result, and a redundant write to the
+      // observed `selectedRange` is what SwiftUI flags as "onChange(of:
+      // AnyTextLayoutCollection) action tried to update multiple times per frame".
+      let reconciled = layoutCollection.reconcileRange(
         selectedRange,
         from: oldLayoutCollection
       )
+      if reconciled != self.selectedRange {
+        self.selectedRange = reconciled
+      }
     }
 
     func setCoordinator(_ coordinator: TextSelectionCoordinator?) {