Fix drag selection freezing over the gutter

Sources/CodeEditTextView/TextView/TextView+Mouse.swift

@@ -34,6 +34,11 @@ extension TextView {
             break
         }
 
+        // Anchor the drag at the press location. Waiting for the first drag event to set this anchors the selection
+        // wherever the pointer had already traveled to by then, which reads as the selection starting a character or
+        // two away from where the user clicked.
+        mouseDragAnchor = clampToTextArea(convert(event.locationInWindow, from: nil))
+
         setUpMouseAutoscrollTimer()
     }
 
@@ -119,11 +124,7 @@ extension TextView {
 
         // We receive global events because our view received the drag event, but we need to clamp the potentially
         // out-of-bounds positions to a position our layout manager can deal with.
-        let locationInWindow = convert(event.locationInWindow, from: nil)
-        let locationInView = CGPoint(
-            x: max(0.0, min(locationInWindow.x, frame.width)),
-            y: max(0.0, min(locationInWindow.y, frame.height))
-        )
+        let locationInView = clampToTextArea(convert(event.locationInWindow, from: nil))
 
         if mouseDragAnchor == nil {
             mouseDragAnchor = locationInView
@@ -147,6 +148,25 @@ extension TextView {
         }
     }
 
+    /// Clamps a point to the region ``TextLayoutManager/textOffsetAtPoint(_:)`` can resolve into a text offset.
+    ///
+    /// Drag events are delivered in global coordinates, so the point can lie well outside the view. The horizontal
+    /// bounds also have to respect ``TextLayoutManager/edgeInsets``: text is laid out inset from the view's edges, and
+    /// any position left of the leading inset — the strip a gutter is drawn over, for instance — resolves to `nil`.
+    ///
+    /// A `nil` offset mid-drag strands the selection at its last resolvable value, so the selection stops responding
+    /// entirely while the pointer sits over the gutter. Clamping into the inset region instead maps those positions to
+    /// the start of a line, which is what AppKit does.
+    func clampToTextArea(_ point: CGPoint) -> CGPoint {
+        let insets = layoutManager.edgeInsets
+        let minX = insets.left
+        let maxX = max(minX, frame.width - insets.right)
+        return CGPoint(
+            x: min(max(point.x, minX), maxX),
+            y: min(max(point.y, 0.0), frame.height)
+        )
+    }
+
     /// Extends the current selection to the offset. Only used when the user shift-clicks a location in the document.
     ///
     /// If the offset is within the selection, trims the selection from the nearest edge (start or end) towards the

Tests/CodeEditTextViewTests/TextViewMouseTests.swift

@@ -0,0 +1,107 @@
+import Testing
+import AppKit
+@testable import CodeEditTextView
+
+@Suite
+@MainActor
+struct TextViewMouseTests {
+    let textView: TextView
+
+    init() {
+        textView = TextView(string: "Lorem Ipsum\nDolor Sit Amet\nConsectetur")
+    }
+
+    /// `layout()` resizes the frame to fit its content (there's no enclosing scroll view to take the size from), so
+    /// the test frame has to be applied afterwards for the view to be genuinely wider than its insets.
+    private func layout(left: CGFloat, right: CGFloat, width: CGFloat = 500) {
+        textView.edgeInsets = HorizontalEdgeInsets(left: left, right: right)
+        textView.layout()
+        textView.frame = NSRect(x: 0, y: 0, width: width, height: 300)
+    }
+
+    /// A drag that leaves the leading inset — the strip a gutter is drawn over — must still resolve to a text
+    /// offset. Otherwise the layout manager returns `nil` and the in-progress selection stops updating.
+    @Test
+    func pointOverLeadingInsetResolvesToTextOffset() {
+        layout(left: 60, right: 0)
+
+        let overGutter = CGPoint(x: 12, y: 2)
+        #expect(textView.layoutManager.textOffsetAtPoint(overGutter) == nil, "Precondition: raw point is unresolvable")
+
+        let clamped = textView.clampToTextArea(overGutter)
+        #expect(clamped.x == 60)
+        #expect(textView.layoutManager.textOffsetAtPoint(clamped) == 0, "Should resolve to the start of line 1")
+    }
+
+    /// Points left of the view entirely — the pointer having been dragged out of the window — clamp the same way.
+    @Test
+    func pointLeftOfViewResolvesToLineStart() {
+        layout(left: 60, right: 0)
+
+        let clamped = textView.clampToTextArea(CGPoint(x: -400, y: 2))
+        #expect(clamped.x == 60)
+        #expect(textView.layoutManager.textOffsetAtPoint(clamped) == 0)
+    }
+
+    @Test
+    func pointRightOfViewClampsInsideTrailingInset() {
+        layout(left: 60, right: 40)
+        #expect(textView.frame.width == 500, "Precondition: the view is wider than its insets")
+
+        let clamped = textView.clampToTextArea(CGPoint(x: 9_000, y: 2))
+        #expect(clamped.x == 460)
+        #expect(textView.layoutManager.textOffsetAtPoint(clamped) != nil)
+    }
+
+    @Test
+    func pointInsideTextAreaIsUnchanged() {
+        layout(left: 60, right: 40)
+
+        let inside = CGPoint(x: 200, y: 2)
+        #expect(textView.clampToTextArea(inside) == inside)
+    }
+
+    @Test
+    func verticalPositionsClampToTheView() {
+        layout(left: 60, right: 0)
+
+        #expect(textView.clampToTextArea(CGPoint(x: 100, y: -500)).y == 0)
+        #expect(textView.clampToTextArea(CGPoint(x: 100, y: 9_000)).y == textView.frame.height)
+    }
+
+    /// With insets wider than the view, the clamp must not produce an inverted range.
+    @Test
+    func degenerateInsetsDoNotInvert() {
+        layout(left: 60, right: 40, width: 50)
+
+        #expect(textView.clampToTextArea(CGPoint(x: 10, y: 2)).x == 60)
+    }
+
+    /// The drag anchor is set on mouse down, not on the first drag event, so a selection starts exactly where the
+    /// user pressed rather than wherever the pointer had traveled to by the time the first drag event arrived.
+    @Test
+    func mouseDownSetsDragAnchor() throws {
+        layout(left: 60, right: 0)
+        #expect(textView.mouseDragAnchor == nil)
+
+        let event = try #require(
+            NSEvent.mouseEvent(
+                with: .leftMouseDown,
+                location: NSPoint(x: 100, y: 2),
+                modifierFlags: [],
+                timestamp: 0,
+                windowNumber: 0,
+                context: nil,
+                eventNumber: 0,
+                clickCount: 1,
+                pressure: 1
+            )
+        )
+        textView.mouseDown(with: event)
+
+        // The view has no window, so the window→view conversion isn't meaningful enough to assert an exact point;
+        // what matters is that the anchor exists before any drag event arrives, and that it's resolvable.
+        let anchor = try #require(textView.mouseDragAnchor)
+        #expect(textView.layoutManager.textOffsetAtPoint(anchor) != nil)
+    }
+}