fix: auto-scroll (for real this time?)

Author: tuxuser

PostCodeSerialMonitor.Tests/AutoScrollTrackerTests.cs

@@ -10,26 +10,39 @@ public void OnScrollChanged_ContentGrowsFasterThanScrollToEndCanCatchUp_StaysAut
     {
         var tracker = new AutoScrollTracker();
 
-        // Simulates a burst of fast-arriving log lines: new content arrives, so the caller asks
-        // whether it should scroll to the end (as MainWindow does on every ItemsRepeater layout
-        // update) and does so, which is what will trigger the ScrollChanged below.
-        Assert.True(tracker.ShouldScrollToEnd());
-
-        // Before that resulting ScrollChanged is processed, more lines already grew the extent
-        // further (104 -> 130) while the offset still reflects the smaller extent it was set
-        // against (54). The user never touched the scrollbar, so auto-scroll should not detach.
+        // Simulates a burst of fast-arriving log lines: ScrollToEnd() was called while the
+        // extent was still 104 (viewport 50), setting the offset to 54. Before that resulting
+        // ScrollChanged is processed, more lines already grew the extent further to 130. The
+        // offset only ever moved forward, so this is not the user scrolling away.
         tracker.OnScrollChanged(offsetY: 54, extentHeight: 130, viewportHeight: 50);
 
         Assert.True(tracker.AutoScroll);
     }
 
     [Fact]
-    public void OnScrollChanged_UserScrollsAwayWithoutScrollToEnd_Detaches()
+    public void OnScrollChanged_UserDragsScrollbarUp_Detaches()
     {
         var tracker = new AutoScrollTracker();
+        tracker.OnScrollChanged(offsetY: 76, extentHeight: 130, viewportHeight: 50); // starts at bottom
 
-        // No ShouldScrollToEnd() call precedes this - the user dragged the scrollbar up themselves.
-        tracker.OnScrollChanged(offsetY: 0, extentHeight: 130, viewportHeight: 50);
+        // The offset moves backward - only a manual scrollbar drag does that.
+        tracker.OnScrollChanged(offsetY: 20, extentHeight: 130, viewportHeight: 50);
+
+        Assert.False(tracker.AutoScroll);
+    }
+
+    [Fact]
+    public void OnScrollChanged_UserDragsAwayAfterSustainedAutoScrolling_StillDetaches()
+    {
+        var tracker = new AutoScrollTracker();
+
+        // Content keeps growing, offset keeps chasing it forward - auto-scroll stays engaged.
+        tracker.OnScrollChanged(offsetY: 50, extentHeight: 104, viewportHeight: 50);
+        tracker.OnScrollChanged(offsetY: 76, extentHeight: 130, viewportHeight: 50);
+        Assert.True(tracker.AutoScroll);
+
+        // The user then grabs the scrollbar and drags it up.
+        tracker.OnScrollChanged(offsetY: 20, extentHeight: 130, viewportHeight: 50);
 
         Assert.False(tracker.AutoScroll);
     }

PostCodeSerialMonitor/Utils/AutoScrollTracker.cs

@@ -8,31 +8,25 @@ public class AutoScrollTracker
     // from item virtualization/resizing so autoscroll doesn't flicker on/off during normal updates.
     private const double BottomThreshold = 4;
 
-    // Set right before we call ScrollToEnd() ourselves, so the ScrollChanged it triggers isn't
-    // mistaken for the user scrolling away (which happens if content keeps growing between our
-    // call and that event, leaving the offset behind a moving extent).
-    private bool _isProgrammaticScroll;
+    private double _lastOffsetY;
 
     public bool AutoScroll { get; private set; } = true;
 
-    public bool ShouldScrollToEnd()
-    {
-        if (!AutoScroll)
-            return false;
-
-        _isProgrammaticScroll = true;
-        return true;
-    }
-
     public void OnScrollChanged(double offsetY, double extentHeight, double viewportHeight)
     {
-        if (_isProgrammaticScroll)
+        if (offsetY < _lastOffsetY)
+        {
+            // The offset moved backward. Our own auto-scroll (ScrollToEnd) never does that -
+            // it only ever moves forward, chasing a growing extent - so this can only be the
+            // user dragging the scrollbar up.
+            AutoScroll = false;
+        }
+        else if (offsetY >= extentHeight - viewportHeight - BottomThreshold)
         {
-            _isProgrammaticScroll = false;
-            return;
+            AutoScroll = true;
         }
 
-        AutoScroll = offsetY >= extentHeight - viewportHeight - BottomThreshold;
+        _lastOffsetY = offsetY;
     }
 
     public void Reset()

PostCodeSerialMonitor/Views/MainWindow.axaml.cs

@@ -51,7 +51,7 @@ private void OnScrollChanged(object? sender, ScrollChangedEventArgs e)
 
     private void OnItemsRepeaterLayoutUpdated(object? sender, EventArgs e)
     {
-        if (_autoScroll.ShouldScrollToEnd() && _scrollViewer != null)
+        if (_autoScroll.AutoScroll && _scrollViewer != null)
         {
             _scrollViewer.ScrollToEnd();
         }