fix(editor): prevent top viewport scroll jump

Sources/SaplingEditor/HybridMarkdownEditor.swift

@@ -689,6 +689,13 @@ private struct NativeMarkdownTextView: NSViewRepresentable {
                 return false
             }
 
+            if anchor.visibleOrigin.y <= 0 {
+                return scrollVisibleOrigin(
+                    NSPoint(x: anchor.visibleOrigin.x, y: 0),
+                    in: textView
+                )
+            }
+
             return scrollVisibleOrigin(
                 NSPoint(
                     x: anchor.visibleOrigin.x,

Tests/SaplingEditorTests/HybridMarkdownLiveEditorHarnessTests.swift

@@ -14,6 +14,19 @@ final class HybridMarkdownLiveEditorHarnessTests: XCTestCase {
         XCTAssertTrue(harness.headingMarkerIsHidden())
     }
 
+    func testClickAtTopOfDocumentDoesNotScrollViewportDown() {
+        let source = (["# Heading", "Opening paragraph"] + (1...80).map { "Line \($0)" })
+            .joined(separator: "\n")
+        let harness = HybridMarkdownLiveEditorHarness(source: source)
+        harness.simulateLaunchFirstResponder()
+        harness.scrollViewport(toY: 0)
+
+        let paragraphLocation = (source as NSString).range(of: "Opening").location
+        harness.setSelectionByMouse(NSRange(location: paragraphLocation, length: 0))
+
+        XCTAssertEqual(harness.viewportOrigin().y, 0, accuracy: 0.001)
+    }
+
     func testLiveParagraphGeometryReturnsAfterClickAndFocusAway() throws {
         let source = """
         # Heading