Cut idle CPU with a busy cluster feed and many windows open

Found by tracing the app with a fake skimmer feed, a fake rigctld and
eight tool windows open (about 3,500 spots on the bandmap).

- Drop expired spots from the one-second spot timer instead of from each
  window's refresh on the UI thread.
- Bandmap keeps the lists On and All return until the spots change, and
  Near searches only the band the frequency is in.
- The Available window replaces its rows at most every two seconds, the
  Score window redraws at most once a second, and the Telnet list takes
  new lines four times a second at most (RateLimited).
- The bandmap leader line pen is made once.

Idle CPU in that setup went from about 33-70% of one core to about 16-25%.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/Nonemm.App/AppSession.cs

@@ -1018,6 +1018,9 @@ public sealed partial class AppSession : IDisposable
     /// put on the bandmap in one go.
     private void FlushSpots()
     {
+        // here and not in the windows: it scans every spot, and it only has to
+        // happen as often as this timer
+        Bandmap.DropOlderThan(DateTime.UtcNow);
         List<Spot> batch;
         lock (arrivingGuard)
         {

src/Nonemm.App/Windows/AvailableWindow.axaml.cs

@@ -37,6 +37,11 @@ public sealed partial class AvailableWindow : RefreshableWindow
     private readonly AppSession session;
     private readonly Action<Frequency, string> tune;
     private readonly DispatcherTimer timer = new() { Interval = TimeSpan.FromSeconds(1) };
+    private readonly RateLimited limited;
+
+    /// What the entry window's changes ask for. The window's own controls and
+    /// its timer call `Refresh` directly.
+    public override void RefreshFromSession() => limited.Request();
 
     /// The bands a click in the summary table limits the list to. Empty is
     /// every band.
@@ -77,10 +82,16 @@ public sealed partial class AvailableWindow : RefreshableWindow
         Spots.PointerExited += (_, _) => Resume();
         Spots.LoadingRow += (_, e) => Paint(e.Row);
         Spots.ContextMenu = BuildMenu();
-        Coalesced changed = new(Refresh);
-        EventHandler spotsChanged = (_, _) => changed.Request();
+        // replacing the grid's rows is the dearest thing this window does, and
+        // spots arrive and the radio moves many times a second
+        limited = new RateLimited(TimeSpan.FromSeconds(2), Refresh);
+        EventHandler spotsChanged = (_, _) => limited.Request();
         session.Bandmap.Changed += spotsChanged;
-        Closed += (_, _) => session.Bandmap.Changed -= spotsChanged;
+        Closed += (_, _) =>
+        {
+            session.Bandmap.Changed -= spotsChanged;
+            limited.Cancel();
+        };
         timer.Tick += (_, _) => OnTick();
         timer.Start();
         Closed += (_, _) => timer.Stop();
@@ -277,7 +288,6 @@ public sealed partial class AvailableWindow : RefreshableWindow
         Frequency? tuned = session.Position?.Frequency;
         IReadOnlyList<ModeCategory> contestModes = session.Position?.Contest.Modes ?? [];
         bool summaryShown = Summary.IsVisible;
-        bandmap.DropOlderThan(now);
         IReadOnlyList<AvailableStation> all = await available.AllAsync();
         Computed computed = await Task.Run(() => Compute(all, now, settings, plan, bandmap, picked, tuned, contestModes, summaryShown));
         if (again)

src/Nonemm.App/Windows/BandmapView.cs

@@ -199,7 +199,7 @@ public sealed class BandmapView : Control
     {
         IBrush colour = ColourOf(label.Spot);
         context.DrawLine(
-            new Pen(Opaque(ink, 0.55), 1),
+            LeaderPen(ink),
             new Point(ScaleX + 2, label.MarkY),
             new Point(LabelX - 2, label.LabelY + (LabelHeight / 2)));
         context.DrawText(
@@ -285,6 +285,20 @@ public sealed class BandmapView : Control
     private IBrush Foreground() =>
         this.FindResource("FormForeground") as IBrush ?? Brushes.Gray;
 
+    private Pen? leaderPen;
+    private IBrush? leaderInk;
+
+    /// The line from the scale to a callsign. Every label has the same one.
+    private Pen LeaderPen(IBrush ink)
+    {
+        if (leaderPen is null || !ReferenceEquals(leaderInk, ink))
+        {
+            leaderPen = new Pen(Opaque(ink, 0.55), 1);
+            leaderInk = ink;
+        }
+        return leaderPen;
+    }
+
     private readonly Dictionary<(IBrush Ink, double Opacity), IBrush> faded = [];
 
     /// Kept, so the same brush comes back each time and the text drawn with it

src/Nonemm.App/Windows/BandmapWindow.axaml.cs

@@ -110,7 +110,6 @@ public sealed partial class BandmapWindow : RefreshableWindow
 
     public override void Refresh()
     {
-        session.Bandmap.DropOlderThan(DateTime.UtcNow);
         Band? band = session.Position is null ? null : Bands.ForFrequency(session.Position.Frequency);
         if (session.Position is null || band is null)
         {

src/Nonemm.App/Windows/RateLimited.cs

@@ -0,0 +1,62 @@
+using System.Diagnostics;
+using Avalonia.Threading;
+
+namespace Nonemm.App.Windows;
+
+/// Runs an action on the UI thread at most once in `every`. The first request
+/// after a quiet spell runs at once; the ones that follow within `every` are
+/// answered by one run when it is up. It is for windows that redraw a whole
+/// list, where following every change costs more than it shows.
+public sealed class RateLimited
+{
+    private readonly TimeSpan every;
+    private readonly Action action;
+    private readonly DispatcherTimer timer = new();
+    private readonly Coalesced onUiThread;
+    private long lastRun = long.MinValue;
+    private bool cancelled;
+
+    public RateLimited(TimeSpan every, Action action)
+    {
+        this.every = every;
+        this.action = action;
+        onUiThread = new Coalesced(Ask);
+        timer.Tick += (_, _) =>
+        {
+            timer.Stop();
+            Run();
+        };
+    }
+
+    /// Safe to call from any thread.
+    public void Request() => onUiThread.Request();
+
+    /// For a window that has closed.
+    public void Cancel()
+    {
+        cancelled = true;
+        timer.Stop();
+    }
+
+    private void Ask()
+    {
+        if (cancelled || timer.IsEnabled)
+        {
+            return;
+        }
+        TimeSpan since = lastRun == long.MinValue ? every : Stopwatch.GetElapsedTime(lastRun);
+        if (since >= every)
+        {
+            Run();
+            return;
+        }
+        timer.Interval = every - since;
+        timer.Start();
+    }
+
+    private void Run()
+    {
+        lastRun = Stopwatch.GetTimestamp();
+        action();
+    }
+}

src/Nonemm.App/Windows/ScoreWindow.axaml.cs

@@ -11,13 +11,21 @@ public sealed partial class ScoreWindow : RefreshableWindow
     private readonly AppSession session;
     private int columnCount = 4;
 
+    private readonly RateLimited limited;
+
     public ScoreWindow(AppSession session)
     {
         this.session = session;
         InitializeComponent();
+        limited = new RateLimited(TimeSpan.FromSeconds(1), Refresh);
+        Closed += (_, _) => limited.Cancel();
         Refresh();
     }
 
+    /// The figures change when a contact is logged, not as the radio is tuned,
+    /// so the entry window's changes redraw this at most once a second.
+    public override void RefreshFromSession() => limited.Request();
+
 
     /// What the table and the total line showed last, so a refresh that
     /// changes nothing leaves the controls alone.

src/Nonemm.App/Windows/TelnetWindow.axaml.cs

@@ -43,7 +43,11 @@ public sealed partial class TelnetWindow : RefreshableWindow
         LoadClusters();
         LoadFilters();
         Listen(session.Cluster);
-        Closed += (_, _) => Forget(session.Cluster);
+        Closed += (_, _) =>
+        {
+            Forget(session.Cluster);
+            flushWaiting?.Cancel();
+        };
         Refresh();
     }
 
@@ -150,12 +154,12 @@ public sealed partial class TelnetWindow : RefreshableWindow
     private void Show(string line, IBrush? colour, bool strong = false) => Dispatcher.UIThread.Post(() =>
     {
         waiting.Add((line, colour, strong));
-        flushWaiting ??= new Coalesced(AddWaiting);
+        flushWaiting ??= new RateLimited(TimeSpan.FromMilliseconds(250), AddWaiting);
         flushWaiting.Request();
     });
 
     private readonly List<(string Line, IBrush? Colour, bool Strong)> waiting = [];
-    private Coalesced? flushWaiting;
+    private RateLimited? flushWaiting;
 
     private void AddWaiting()
     {