From 261c93c14ac60120bb1ff3711430a8451ced56be Mon Sep 17 00:00:00 2001
From: Arnaud Mengus <44212923+isontheline@users.noreply.github.com>
Date: Mon, 13 Jul 2026 10:57:23 +0200
Subject: [PATCH] Fixing https://github.com/simonbs/Runestone/issues/415

---
 .../Core/LineMovementController.swift         | 21 +++++++++++++++++--
 .../Core/TextInputStringTokenizer.swift       | 19 +++++++++++++++--
 2 files changed, 36 insertions(+), 4 deletions(-)

diff --git a/Sources/Runestone/TextView/Core/LineMovementController.swift b/Sources/Runestone/TextView/Core/LineMovementController.swift
index 60638eba5..f7a0fdafb 100644
--- a/Sources/Runestone/TextView/Core/LineMovementController.swift
+++ b/Sources/Runestone/TextView/Core/LineMovementController.swift
@@ -61,6 +61,12 @@ private extension LineMovementController {
             return location
         }
         let lineLocalLocation = max(min(location - line.location, line.data.totalLength), 0)
+        // After an edit the line fragment tree is reset and only repopulated for lines laid out
+        // by the layout manager, so the query below may otherwise hit an empty tree.
+        // See https://github.com/simonbs/Runestone/issues/415
+        if lineController.numberOfLineFragments == 0 || !lineController.isFinishedTypesetting {
+            lineController.prepareToDisplayString(toLocation: lineLocalLocation, syntaxHighlightAsynchronously: true)
+        }
         guard let lineFragmentNode = lineController.lineFragmentNode(containingCharacterAt: lineLocalLocation) else {
             return location
         }
@@ -78,7 +84,7 @@ private extension LineMovementController {
             return locationForMovingDownwards(lineOffset: lineOffset, fromLocation: location, inLineFragmentAt: lineFragmentIndex, of: line)
         } else {
             // lineOffset is 0 so we shouldn't change the line
-            let lineController = lineControllerStorage.getOrCreateLineController(for: line)
+            let lineController = preparedLineController(for: line)
             let destinationLineFragmentNode = lineController.lineFragmentNode(atIndex: lineFragmentIndex)
             let lineLocation = line.location
             let preferredLocation = lineLocation + destinationLineFragmentNode.location + location
@@ -132,7 +138,18 @@ private extension LineMovementController {
     }
 
     private func numberOfLineFragments(in line: DocumentLineNode) -> Int {
-        let lineController = lineControllerStorage.getOrCreateLineController(for: line)
+        let lineController = preparedLineController(for: line)
         return lineController.numberOfLineFragments
     }
+
+    // Ensures the entire line is typeset so its line fragments can be counted and queried.
+    // After an edit the line fragment tree is reset and only repopulated for lines laid out
+    // by the layout manager. See https://github.com/simonbs/Runestone/issues/415
+    private func preparedLineController(for line: DocumentLineNode) -> LineController {
+        let lineController = lineControllerStorage.getOrCreateLineController(for: line)
+        if lineController.numberOfLineFragments == 0 || !lineController.isFinishedTypesetting {
+            lineController.prepareToDisplayString(toLocation: line.data.totalLength, syntaxHighlightAsynchronously: true)
+        }
+        return lineController
+    }
 }
diff --git a/Sources/Runestone/TextView/Core/TextInputStringTokenizer.swift b/Sources/Runestone/TextView/Core/TextInputStringTokenizer.swift
index 12f45fab2..629bd5cbb 100644
--- a/Sources/Runestone/TextView/Core/TextInputStringTokenizer.swift
+++ b/Sources/Runestone/TextView/Core/TextInputStringTokenizer.swift
@@ -49,6 +49,18 @@ final class TextInputStringTokenizer: UITextInputStringTokenizer {
 
 // MARK: - Lines
 private extension TextInputStringTokenizer {
+    // Ensures the line is typeset far enough that its line fragment tree can answer queries
+    // at lineLocalLocation. After an edit the line fragment tree is reset and only repopulated
+    // for lines laid out by the layout manager, so UIKit's selection queries (e.g. during paste)
+    // may otherwise hit an empty tree. See https://github.com/simonbs/Runestone/issues/415
+    private func preparedLineController(for line: DocumentLineNode, toLocation lineLocalLocation: Int) -> LineController {
+        let lineController = lineControllerStorage.getOrCreateLineController(for: line)
+        if lineController.numberOfLineFragments == 0 || !lineController.isFinishedTypesetting {
+            lineController.prepareToDisplayString(toLocation: lineLocalLocation, syntaxHighlightAsynchronously: true)
+        }
+        return lineController
+    }
+
     private func isPosition(_ position: UITextPosition, atLineBoundaryInDirection direction: UITextDirection) -> Bool {
         guard let indexedPosition = position as? IndexedPosition else {
             return false
@@ -59,10 +71,10 @@ private extension TextInputStringTokenizer {
         }
         let lineLocation = line.location
         let lineLocalLocation = location - lineLocation
-        let lineController = lineControllerStorage.getOrCreateLineController(for: line)
         guard lineLocalLocation >= 0 && lineLocalLocation <= line.data.totalLength else {
             return false
         }
+        let lineController = preparedLineController(for: line, toLocation: lineLocalLocation)
         guard let lineFragmentNode = lineController.lineFragmentNode(containingCharacterAt: lineLocalLocation) else {
             return false
         }
@@ -86,9 +98,12 @@ private extension TextInputStringTokenizer {
         guard let line = lineManager.line(containingCharacterAt: location) else {
             return nil
         }
-        let lineController = lineControllerStorage.getOrCreateLineController(for: line)
         let lineLocation = line.location
         let lineLocalLocation = location - lineLocation
+        guard lineLocalLocation >= 0 && lineLocalLocation <= line.data.totalLength else {
+            return nil
+        }
+        let lineController = preparedLineController(for: line, toLocation: lineLocalLocation)
         guard let lineFragmentNode = lineController.lineFragmentNode(containingCharacterAt: lineLocalLocation) else {
             return nil
         }