Fix highlight region destruction during synchronous layout

Sources/Litext/LTXLabel/Highlight/LTXHighlightRegion.swift

@@ -18,6 +18,10 @@ public class LTXHighlightRegion {
         self.stringRange = stringRange
     }
 
+    // Avoid the isolated-deinit runtime crash when synchronous layout releases a region.
+    // https://github.com/swiftlang/swift/issues/85663
+    nonisolated deinit {}
+
     func addRect(_ rect: CGRect) {
         #if canImport(UIKit)
             rects.append(NSValue(cgRect: rect))

Tests/MarkdownViewTests/HighlightRegionLifetimeTests.swift

@@ -0,0 +1,30 @@
+import Foundation
+import XCTest
+@testable import Litext
+
+@MainActor
+final class HighlightRegionLifetimeTests: XCTestCase {
+    private enum LayoutContext {
+        @TaskLocal static var value = "outside"
+    }
+
+    // Keep this synchronous: UIKit layout and observation callbacks need not run in a Swift task.
+    func testSynchronousReleaseInsideTaskLocalScope() {
+        for _ in 0 ..< 100 {
+            LayoutContext.$value.withValue("layout") {
+                autoreleasepool {
+                    var regions = [0: LTXHighlightRegion(
+                        attributes: [NSAttributedString.Key("link"): "https://example.com"],
+                        stringRange: NSRange(location: 0, length: 4)
+                    )]
+                    weak var region: LTXHighlightRegion?
+                    region = regions[0]
+                    regions.removeAll()
+                    XCTAssertNil(region)
+                }
+                XCTAssertEqual(LayoutContext.value, "layout")
+            }
+            XCTAssertEqual(LayoutContext.value, "outside")
+        }
+    }
+}