fix: Autoscroll

Author: tuxuser

PostCodeSerialMonitor.Tests/AutoScrollTrackerTests.cs

@@ -0,0 +1,36 @@
+using PostCodeSerialMonitor.Utils;
+using Xunit;
+
+namespace PostCodeSerialMonitor.Tests;
+
+public class AutoScrollTrackerTests
+{
+    [Fact]
+    public void OnScrollChanged_ContentGrowsFasterThanScrollToEndCanCatchUp_StaysAutoScrolled()
+    {
+        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.
+        tracker.OnScrollChanged(offsetY: 54, extentHeight: 130, viewportHeight: 50);
+
+        Assert.True(tracker.AutoScroll);
+    }
+
+    [Fact]
+    public void OnScrollChanged_UserScrollsAwayWithoutScrollToEnd_Detaches()
+    {
+        var tracker = new AutoScrollTracker();
+
+        // No ShouldScrollToEnd() call precedes this - the user dragged the scrollbar up themselves.
+        tracker.OnScrollChanged(offsetY: 0, extentHeight: 130, viewportHeight: 50);
+
+        Assert.False(tracker.AutoScroll);
+    }
+}

PostCodeSerialMonitor/Utils/AutoScrollTracker.cs

@@ -0,0 +1,42 @@
+namespace PostCodeSerialMonitor.Utils;
+
+// Decides whether a log view should keep auto-scrolling to the bottom as new
+// entries arrive, versus staying put because the user scrolled away.
+public class AutoScrollTracker
+{
+    // Distance from the bottom (in pixels) still considered "at the bottom", to absorb layout jitter
+    // 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;
+
+    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)
+        {
+            _isProgrammaticScroll = false;
+            return;
+        }
+
+        AutoScroll = offsetY >= extentHeight - viewportHeight - BottomThreshold;
+    }
+
+    public void Reset()
+    {
+        AutoScroll = true;
+    }
+}

PostCodeSerialMonitor/Views/MainWindow.axaml.cs

@@ -9,7 +9,7 @@ namespace PostCodeSerialMonitor.Views;
 
 public partial class MainWindow : Window
 {
-    private bool _autoScroll = true;
+    private readonly AutoScrollTracker _autoScroll = new();
     private ScrollViewer? _scrollViewer;
     private ItemsRepeater? _itemsRepeater;
 
@@ -38,33 +38,28 @@ protected override void OnLoaded(RoutedEventArgs e)
         }
     }
 
-    // Distance from the bottom (in pixels) still considered "at the bottom", to absorb layout jitter
-    // from item virtualization/resizing so autoscroll doesn't flicker on/off during normal updates.
-    private const double BottomThreshold = 4;
-
     private void OnScrollChanged(object? sender, ScrollChangedEventArgs e)
     {
         if (_scrollViewer == null) return;
 
-        var atBottom = _scrollViewer.Offset.Y >= _scrollViewer.Extent.Height - _scrollViewer.Viewport.Height - BottomThreshold;
-        _autoScroll = atBottom;
+        _autoScroll.OnScrollChanged(_scrollViewer.Offset.Y, _scrollViewer.Extent.Height, _scrollViewer.Viewport.Height);
         if (AutoScrollButton != null)
         {
-            AutoScrollButton.IsVisible = !atBottom;
+            AutoScrollButton.IsVisible = !_autoScroll.AutoScroll;
         }
     }
 
     private void OnItemsRepeaterLayoutUpdated(object? sender, EventArgs e)
     {
-        if (_autoScroll && _scrollViewer != null)
+        if (_autoScroll.ShouldScrollToEnd() && _scrollViewer != null)
         {
             _scrollViewer.ScrollToEnd();
         }
     }
 
     private void OnAutoScrollButtonClick(object? sender, RoutedEventArgs e)
     {
-        _autoScroll = true;
+        _autoScroll.Reset();
         if (AutoScrollButton != null)
         {
             AutoScrollButton.IsVisible = false;