iOS: NSRangeException crash drawing truncated <Text> when the ellipsis boundary splits a composed character cluster (processTruncatedAttributedText)

Beginne mit processTruncatedAttributedText in ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm und reproduziere den Absturz mit dem verknüpften Snack auf einem iOS-Gerät oder Simulator. Verfolge die im Issue beschriebene Enumeration, Attributänderungen und den Highlight-Pass und verifiziere anschließend, dass der Breiten-Sweep für zusammengesetzte Zeichen beim Zeichnen keine NSRangeException mehr auslöst.

Description

Rendering a <Text numberOfLines={1}> whose content contains composed character clusters (stacked combining marks, e.g. Zalgo-style user display names) crashes the app on iOS with an uncatchable NSRangeException when the truncation ellipsis boundary falls inside one of those clusters:

NSRangeException: *** -[NSConcreteTextStorage attributesAtIndex:longestEffectiveRange:inRange:]: Range or index out of bounds
(also observed as NSMutableRLEArray objectAtIndex:effectiveRange:: Out of bounds)

This is a production crash for us: any user whose friend/chat contact has such a display name gets a deterministic crash on every visit to the screen rendering it. Because the throw happens on the main thread inside Core Animation's draw pass, it is not catchable from JS and always aborts the app.

Root cause

The problem is in processTruncatedAttributedText in ReactCommon/react/renderer/textlayoutmanager/platform/ios/react/renderer/textlayoutmanager/RCTTextLayoutManager.mm (introduced for #37926, unchanged on main at time of filing):

1. drawAttributedString:paragraphAttributes:frame:drawHighlightPath: computes glyphRange, then calls processTruncatedAttributedText.
2. When maximumNumberOfLines > 0 and truncation is active, that method mutates the NSTextStorage (removeAttribute: / addAttribute: for foreground/background color on the truncated character range) inside enumerateLineFragmentsForGlyphRange: — mutating a text storage during its own layout manager's enumeration is unsupported by TextKit.
3. Each mutation triggers attribute fixing (fixAttributesInRange), which snaps attribute runs to composed-character-sequence boundaries. When the truncated character range starts mid-cluster, the runs shift relative to the layout manager's already-computed glyph→character mapping.
4. The subsequent drawGlyphsForGlyphRange: / the highlight pass (characterRangeForGlyphRange: + enumerateAttribute:inRange:) query the storage with stale ranges → NSRangeException → SIGABRT.

Suggested fix direction
Collect the truncated character ranges during enumerateLineFragmentsForGlyphRange: and apply the attribute edits after the enumeration (wrapped in beginEditing/endEditing); expand ranges with rangeOfComposedCharacterSequencesForRange: before mutating; and clamp with NSIntersectionRange against textStorage.length before enumerateAttribute:inRange: in the highlight pass.

Steps to reproduce

1. Open the Snack on an iOS device or simulator (not web): https://snack.expo.dev/@normanwilde.equals/nsrangeexception-crash
2. Scroll through the list.
3. The app crashes with NSRangeException once a row's width truncates the string mid-cluster.

The Snack renders this production-sourced display name at container widths 20→200pt in 0.5pt steps with numberOfLines={1}:

◍🥝۪〬.࠭⤿𝓚𝔀𝓜◍🥭۪〬

(22 code points / ~8 visible glyphs: combining marks — including U+20DD COMBINING ENCLOSING CIRCLE and Arabic/Meetei Mayek marks — stacked on symbol and emoji bases, plus Mathematical Script letters.)
The width sweep matters: the crash only fires at widths where the ellipsis cuts inside a composed cluster, so a single fixed width usually misses — which is why this is hard to hit in a trivial test yet deterministic in real layouts.

React Native Version

0.85.3

Affected Platforms

Runtime - iOS

Output of npx @react-native-community/cli info

System:
  OS: macOS 26.5
  CPU: (10) arm64 Apple M4
  Memory: 215.69 MB / 16.00 GB
  Shell:
    version: "5.9"
    path: /bin/zsh
Binaries:
  Node:
    version: 22.22.2
    path: ~/.nvm/versions/node/v22.22.2/bin/node
  Yarn:
    version: 4.7.0
    path: /opt/homebrew/bin/yarn
  npm:
    version: 10.9.7
    path: ~/.nvm/versions/node/v22.22.2/bin/npm
  Watchman:
    version: 2024.07.29.00
    path: /opt/homebrew/bin/watchman
Managers:
  CocoaPods:
    version: 1.16.2
    path: ~/.rbenv/shims/pod
SDKs:
  iOS SDK:
    Platforms:
      - DriverKit 25.5
      - iOS 26.5
      - macOS 26.5
      - tvOS 26.5
      - visionOS 26.5
      - watchOS 26.5
IDEs:
  Xcode:
    version: 26.6/17F113
    path: /usr/bin/xcodebuild
Languages:
  Ruby:
    version: 3.3.4
npmPackages:
  react:
    installed: 19.2.3
    wanted: 19.2.3
  react-native:
    installed: 0.85.3
    wanted: 0.85.3

Stacktrace or Logs

NSRangeException: *** -[NSConcreteTextStorage attributesAtIndex:longestEffectiveRange:inRange:]: Range or index out of bounds
  0   CoreFoundation       __exceptionPreprocess
  1   libobjc.A.dylib      objc_exception_throw
  2   UIFoundation         (NSLayoutManager internals)
  3   UIFoundation         (NSLayoutManager internals)
  4   React                -[RCTTextLayoutManager drawAttributedString:paragraphAttributes:frame:drawHighlightPath:] + 616   (RCTTextLayoutManager.mm:98)
  5   React                -[RCTParagraphTextView drawRect:] + 548   (RCTParagraphComponentView.mm:409)
  6   UIKitCore            (CALayerDelegate drawLayer:inContext:)
  7   UIKitCore            UIGraphicsPushContext
  9   QuartzCore           CABackingStoreUpdate_
 12   QuartzCore           CA::Layer::layout_and_display_if_needed
 14   QuartzCore           CA::Transaction::commit()
 ...  (main run loop / UIApplicationMain)

MANDATORY Reproducer

https://snack.expo.dev/@normanwilde.equals/nsrangeexception-crash

Screenshots and Videos

No response