Fix highlight region destruction during synchronous layout

[gtokman]

Problem

Synchronous UIKit text layout can abort while LTXTextLayout.updateHighlightRegions() releases a highlight region. Litext's default MainActor isolation gives this otherwise ordinary Swift class an implicit isolated destructor. On affected runtimes, destruction inside a synchronous task-local scope enters swift_task_deinitOnExecutorImpl and fails in TaskLocal::StopLookupScope with an invalid free (swiftlang/swift#85663).

Fix

Declare an empty nonisolated deinit on LTXHighlightRegion. Its properties and methods retain their existing isolation; destruction has no custom actor-bound cleanup and no longer calls the isolated-deinit runtime. Add a synchronous XCTest that clears a region dictionary inside a task-local scope, checks immediate release, and verifies that the surrounding task-local value is restored.

Validation

• Compiled the actual region source into an optimized iOS simulator app with Swift 6.3.3 and default MainActor isolation. On iOS 26.2, the original class terminated with SIGABRT; the patched class completed 100 releases and restored task-local values successfully.
• Generated SIL confirms that the patched destructor no longer references swift_task_deinitOnExecutor.
• Built the full Litext source target and ran the new XCTest in an isolated macOS SwiftPM harness: one test passed.
• No xcodebuild, full MarkdownView suite, or physical-device test was run. The exact reported iOS 26.3 runtime is not installed locally.

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Project	Deployment	Actions	Updated
markdown-view	Ready	Preview, v0	Oct 9, 2026 6:49pm UTC