Preserve explicit wheel scrollback against passive follow

Author: lawrencecchen

Sources/GhosttyTerminalView.swift

@@ -6052,6 +6052,7 @@ class GhosttyNSView: NSView, NSUserInterfaceValidations {
     }
 
     override func scrollWheel(with event: NSEvent) {
+        NotificationCenter.default.post(name: .ghosttyDidReceiveWheelScroll, object: self)
         guard let surface = surface else { return }
         lastScrollEventTime = CACurrentMediaTime()
         Self.focusLog("scrollWheel: surface=\(terminalSurface?.id.uuidString ?? "nil") firstResponder=\(String(describing: window?.firstResponder))")
@@ -6453,6 +6454,7 @@ enum GhosttyNotificationKey {
 extension Notification.Name {
     static let ghosttyDidUpdateScrollbar = Notification.Name("ghosttyDidUpdateScrollbar")
     static let ghosttyDidUpdateCellSize = Notification.Name("ghosttyDidUpdateCellSize")
+    static let ghosttyDidReceiveWheelScroll = Notification.Name("ghosttyDidReceiveWheelScroll")
     static let ghosttySearchFocus = Notification.Name("ghosttySearchFocus")
     static let ghosttyConfigDidReload = Notification.Name("ghosttyConfigDidReload")
     static let ghosttyDefaultBackgroundDidChange = Notification.Name("ghosttyDefaultBackgroundDidChange")
@@ -6586,6 +6588,8 @@ final class GhosttySurfaceScrollView: NSView {
     /// When true, auto-scroll should be suspended to prevent the "doomscroll" bug
     /// where the terminal fights the user's scroll position.
     private var userScrolledAwayFromBottom = false
+    private var pendingExplicitWheelScroll = false
+    private var allowExplicitScrollbarSync = false
     /// Threshold in points from bottom to consider "at bottom" (allows for minor float drift)
     private static let scrollToBottomThreshold: CGFloat = 5.0
     private var isActive = true
@@ -7011,6 +7015,14 @@ final class GhosttySurfaceScrollView: NSView {
             self?.handleScrollbarUpdate(notification)
         })
 
+        observers.append(NotificationCenter.default.addObserver(
+            forName: .ghosttyDidReceiveWheelScroll,
+            object: surfaceView,
+            queue: .main
+        ) { [weak self] _ in
+            self?.pendingExplicitWheelScroll = true
+        })
+
         observers.append(NotificationCenter.default.addObserver(
             forName: .ghosttySearchFocus,
             object: nil,
@@ -9013,26 +9025,21 @@ final class GhosttySurfaceScrollView: NSView {
                     userScrolledAwayFromBottom = false
                 }
 
-                // Only auto-scroll if user hasn't manually scrolled away from bottom
-                // or if we're following terminal output (scrollbar shows we're at bottom)
-                let shouldAutoScroll = !userScrolledAwayFromBottom ||
-                    (scrollbar.offset + scrollbar.len >= scrollbar.total)
+                // Passive bottom packets should not override an explicit scrollback review,
+                // but the first scrollbar packet caused by the user's own wheel input should
+                // still move the viewport to the requested scrollback position.
+                let shouldAutoScroll = !userScrolledAwayFromBottom || allowExplicitScrollbarSync
 
                 if shouldAutoScroll && !pointApproximatelyEqual(currentOrigin, targetOrigin) {
-#if DEBUG
-                    logDragGeometryChange(
-                        event: "scrollOrigin",
-                        old: currentOrigin,
-                        new: targetOrigin
-                    )
-#endif
                     scrollView.contentView.scroll(to: targetOrigin)
                     didChangeGeometry = true
                 }
                 lastSentRow = Int(scrollbar.offset)
             }
         }
 
+        allowExplicitScrollbarSync = false
+
         if didChangeGeometry {
             scrollView.reflectScrolledClipView(scrollView.contentView)
         }
@@ -9068,6 +9075,11 @@ final class GhosttySurfaceScrollView: NSView {
         guard let scrollbar = notification.userInfo?[GhosttyNotificationKey.scrollbar] as? GhosttyScrollbar else {
             return
         }
+        if pendingExplicitWheelScroll {
+            userScrolledAwayFromBottom = scrollbar.offset + scrollbar.len < scrollbar.total
+            allowExplicitScrollbarSync = true
+            pendingExplicitWheelScroll = false
+        }
         surfaceView.scrollbar = scrollbar
         synchronizeScrollView()
     }