1.7.2: donut highlights pile up and slices fly apart when frames come late

CHANGELOG.md

@@ -6,6 +6,15 @@ version it publishes under "What's Changed" in the GitHub release notes, above t
 Add a `## <version>` section before tagging a release. Write what someone using Filee notices: what's new, where to
 find it, what it does and doesn't do.
 
+## 1.7.2
+
+### Fixes
+
+- When the computer was busy (a screen recorder running, a slow graphics card), slices of the donut could stay lit
+  after the pointer left them, pile up in the accent colour and fly apart as the donut closed. A late frame moved the
+  highlight further than it should, and every following frame made it worse. The animations now stay steady however
+  late the frames come. Turning on "Reduce animations" was the workaround until now.
+
 ## 1.7.1
 
 ### Fixes

Directory.Build.props

@@ -14,7 +14,7 @@
          parameters of a member is fine (CS1573). -->
     <NoWarn>$(NoWarn);CS1591;CS1573</NoWarn>
 
-    <Version>1.7.1</Version>
+    <Version>1.7.2</Version>
     <Product>Filee</Product>
     <Authors>Filee contributors</Authors>
     <Copyright>Copyright (c) Filee contributors</Copyright>

src/Filee.App/Controls/DonutMenu.cs

@@ -478,11 +478,22 @@ private struct Spring
         public double Value;
         public double Velocity;
 
+        /// <summary>
+        /// Longest time one integration step covers. With these stiffnesses a single step of a late frame (50 ms)
+        /// overshoots more each frame instead of settling: slices stayed lit and flew apart on a busy machine.
+        /// </summary>
+        private const double MaxStep = 1 / 120.0;
+
         public bool Step(double target, double dt, double stiffness = 320, double damping = 20)
         {
-            var force = stiffness * (target - Value) - damping * Velocity;
-            Velocity += force * dt;
-            Value += Velocity * dt;
+            var steps = Math.Max(1, (int)Math.Ceiling(dt / MaxStep));
+            var h = dt / steps;
+            for (var i = 0; i < steps; i++)
+            {
+                var force = stiffness * (target - Value) - damping * Velocity;
+                Velocity += force * h;
+                Value += Velocity * h;
+            }
             var settled = Math.Abs(target - Value) < 0.001 && Math.Abs(Velocity) < 0.01;
             if (settled)
             {

tests/Filee.App.Tests/DonutHighlightTests.cs

@@ -91,6 +91,34 @@ public void A_highlight_settles_without_dipping_below_zero()
         Assert.Equal(0, donut.LitAmount(1), 3);
     }
 
+    [AvaloniaFact]
+    public void Highlights_settle_when_frames_come_late()
+    {
+        TestServices.EnsureInitialized("ko");
+        Filee.App.Services.Motion.Configure(false, () => false);
+        var donut = new DonutMenu
+        {
+            Items = new[] { "PNG", "JPG", "WEBP", "PDF" }.Select(l => new DonutItem { Id = l, Label = l }).ToList(),
+        };
+        // Every frame as late as the loop allows (a busy GPU, a screen recorder): the pointer crosses the slices.
+        const double late = 0.05;
+        for (var slice = 0; slice < 4; slice++)
+        {
+            donut.SetExternalHighlight(slice);
+            for (var i = 0; i < 6; i++)
+                donut.Advance(late);
+        }
+        for (var i = 0; i < 60; i++)
+        {
+            donut.Advance(late);
+            for (var slice = 0; slice < 3; slice++)
+                Assert.InRange(donut.LitAmount(slice), 0, 1);
+        }
+
+        Assert.All(Enumerable.Range(0, 3), slice => Assert.Equal(0, donut.LitAmount(slice), 3));
+        Assert.Equal(1, donut.LitAmount(3), 3);
+    }
+
     [AvaloniaFact] // switching the setting restyles the open windows, which belong to the UI thread
     public void Following_the_system_reads_it_again_when_a_window_opens()
     {