Stabilize toolbar scroll position during resize

Author: cavs1910

CHANGELOG.md

@@ -2,6 +2,12 @@
 
 All notable changes to CodeViewerKit are documented in this file.
 
+## 0.6.5 - 2026-08-27
+
+- prevent AppKit's clip view from independently changing the toolbar content
+  inset during macOS window resizes, so the first visible code line and gutter
+  number remain below the toolbar;
+
 ## 0.6.4 - 2026-08-27
 
 - expose each registered grammar's canonical identifier and aliases so a

Sources/CodeViewerKit/CodeTextView.swift

@@ -406,6 +406,10 @@ private final class MacCodeTextContainer: NSView {
         scrollView.hasHorizontalScroller = true
         scrollView.autohidesScrollers = true
         scrollView.scrollerStyle = .legacy
+        // The scroll view and its clip view adjust insets independently. Own
+        // both values so AppKit cannot move the clip bounds during live resize.
+        scrollView.automaticallyAdjustsContentInsets = false
+        scrollView.contentView.automaticallyAdjustsContentInsets = false
         scrollView.documentView = textView
 
         textView.drawsBackground = false
@@ -523,8 +527,8 @@ private final class MacCodeTextContainer: NSView {
         )
         guard abs(scrollView.contentInsets.top - topInset) > 0.5 else { return }
 
-        scrollView.automaticallyAdjustsContentInsets = false
         scrollView.contentInsets.top = topInset
+        scrollView.contentView.contentInsets.top = topInset
     }
 
     private func updateLineWrapping(_ lineWrapping: CodeLineWrapping) {

Tests/CodeViewerKitTests/CodeTextCoreTests.swift

@@ -131,6 +131,63 @@ final class CodeTextCoreTests: XCTestCase {
             CGPoint(x: 7, y: 140)
         )
     }
+
+    @MainActor
+    func testMacViewerKeepsFirstLineBelowToolbarWhileWindowResizes() throws {
+        let highlights = CodeHighlightStore(grammars: [])
+        let sourceCode = (1...200)
+            .map {
+                "let line\($0) = \($0) // " + String(repeating: "x", count: 60)
+            }
+            .joined(separator: "\n")
+        let hostingView = NSHostingView(
+            rootView: CodeViewer(
+                documentID: "resize-test",
+                sourceCode: sourceCode,
+                highlightStore: highlights,
+                language: "plain"
+            )
+            .ignoresSafeArea()
+        )
+        let window = NSWindow(
+            contentRect: NSRect(x: 0, y: 0, width: 700, height: 500),
+            styleMask: [.titled, .resizable, .fullSizeContentView],
+            backing: .buffered,
+            defer: false
+        )
+        window.titleVisibility = .hidden
+        window.titlebarAppearsTransparent = true
+        window.toolbar = NSToolbar(identifier: "CodeViewerKit.ResizeTest")
+        window.contentView = hostingView
+
+        hostingView.layoutSubtreeIfNeeded()
+
+        let scrollView = try XCTUnwrap(
+            hostingView.firstDescendant(ofType: NSScrollView.self)
+        )
+        XCTAssertGreaterThan(scrollView.contentInsets.top, 0)
+        XCTAssertFalse(scrollView.automaticallyAdjustsContentInsets)
+        XCTAssertFalse(scrollView.contentView.automaticallyAdjustsContentInsets)
+        XCTAssertEqual(
+            scrollView.contentView.bounds.minY + scrollView.contentInsets.top,
+            0,
+            accuracy: 0.5
+        )
+
+        for size in [
+            NSSize(width: 300, height: 350),
+            NSSize(width: 900, height: 600)
+        ] {
+            window.setContentSize(size)
+            hostingView.layoutSubtreeIfNeeded()
+
+            XCTAssertEqual(
+                scrollView.contentView.bounds.minY + scrollView.contentInsets.top,
+                0,
+                accuracy: 0.5
+            )
+        }
+    }
     #endif
 
     func testPlainTextStyleUsesAnAppearanceAwareDefault() {
@@ -175,3 +232,22 @@ final class CodeTextCoreTests: XCTestCase {
         #endif
     }
 }
+
+#if os(macOS)
+private extension NSView {
+    func firstDescendant<ViewType: NSView>(
+        ofType type: ViewType.Type
+    ) -> ViewType? {
+        if let match = self as? ViewType {
+            return match
+        }
+
+        for subview in subviews {
+            if let match = subview.firstDescendant(ofType: type) {
+                return match
+            }
+        }
+        return nil
+    }
+}
+#endif