Fix stale text position handling

Author: whitneyland

Sources/Textual/Internal/TextInteraction/Shared/TextLayout/TextLayoutCollection+Geometry.swift

@@ -124,6 +124,10 @@
     }
 
     func isPositionAtBlockBoundary(_ position: TextPosition) -> Bool {
+      guard contains(position) else {
+        return false
+      }
+
       if position
         == TextPosition(
           indexPath: .init(layout: position.indexPath.layout),

Sources/Textual/Internal/TextInteraction/Shared/TextLayout/TextLayoutCollection+Positioning.swift

@@ -29,7 +29,9 @@
     }
 
     func position(from position: TextPosition, offset: Int) -> TextPosition? {
-      let from = characterIndex(at: position)
+      guard let from = characterIndex(at: position) else {
+        return nil
+      }
       let target = from + offset
 
       guard (0...stringLength).contains(target) else {
@@ -59,13 +61,38 @@
       return self.position(at: layout, localCharacterIndex: localTarget)
     }
 
-    func characterIndex(at position: TextPosition) -> Int {
+    func characterIndex(at position: TextPosition) -> Int? {
+      guard contains(position) else {
+        return nil
+      }
       let base = layouts.prefix(position.indexPath.layout)
         .map(\.attributedString.length)
         .reduce(0, +)
       return base + localCharacterIndex(at: position)
     }
 
+    func contains(_ position: TextPosition) -> Bool {
+      let indexPath = position.indexPath
+      guard
+        indexPath.count == 4,
+        layouts.indices.contains(indexPath.layout)
+      else {
+        return false
+      }
+
+      let layout = layouts[indexPath.layout]
+      guard layout.lines.indices.contains(indexPath.line) else {
+        return false
+      }
+
+      let line = layout.lines[indexPath.line]
+      guard line.runs.indices.contains(indexPath.run) else {
+        return false
+      }
+
+      return line.runs[indexPath.run].slices.indices.contains(indexPath.runSlice)
+    }
+
     func localCharacterIndex(at position: TextPosition) -> Int {
       let range = localCharacterRange(at: position.indexPath)
       switch position.affinity {

Sources/Textual/Internal/TextInteraction/Shared/TextSelectionModel.swift

@@ -111,7 +111,13 @@
     }
 
     func offset(from: TextPosition, to: TextPosition) -> Int {
-      layoutCollection.characterIndex(at: to) - layoutCollection.characterIndex(at: from)
+      guard
+        let fromIndex = layoutCollection.characterIndex(at: from),
+        let toIndex = layoutCollection.characterIndex(at: to)
+      else {
+        return 0
+      }
+      return toIndex - fromIndex
     }
 
     func firstRect(for range: TextRange) -> CGRect {

Tests/TextualTests/Internal/TextInteraction/TextSelectionModelTests.swift

@@ -198,6 +198,23 @@
       #expect(offset == -38)
     }
 
+    @Test
+    func stalePositionsAfterLayoutBecomesEmpty() throws {
+      // given
+      let model = try TextSelectionModel(fixtureName: "two-paragraphs-bidi")
+      let staleStart = model.startPosition
+      let staleEnd = model.endPosition
+
+      // when
+      model.setLayoutCollection(EmptyTextLayoutCollection())
+
+      // then
+      #expect(model.position(from: staleStart, offset: 0) == nil)
+      #expect(model.offset(from: staleStart, to: staleEnd) == 0)
+      #expect(model.isPositionAtBlockBoundary(staleStart) == false)
+      #expect(model.isPositionAtBlockBoundary(staleEnd) == false)
+    }
+
     @Test
     func firstRectCollapsedRange() throws {
       // given