Fix #916 — Memory tab tooltip stops working after returning to tab

WPF Popup with PlacementTarget = chart can wedge when the user navigates
away from a TabItem mid-hover: TabControl unloads the parent without firing
MouseLeave on the chart, so _popup.IsOpen stays true with a stale anchor.
On return, OnMouseMove sets IsOpen = true but it is already true — the
assignment is a no-op and the popup never re-anchors or appears.

Memory tab is the most visible victim because it has 6 charts inside a
nested TabControl, multiplying the chance of a wedged popup, but the bug
is general to ChartHoverHelper.

- Force _popup.IsOpen = false on chart Loaded / Unloaded / IsVisibleChanged.
- In OnMouseMove, toggle IsOpen off then on so WPF re-evaluates placement
  even when the popup believes it is already open.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: erikdarlingdata

Dashboard/Helpers/ChartHoverHelper.cs

@@ -53,6 +53,16 @@ public ChartHoverHelper(ScottPlot.WPF.WpfPlot chart, string unit)
 
         chart.MouseMove += OnMouseMove;
         chart.MouseLeave += OnMouseLeave;
+
+        /* Tab switching can leave the popup wedged: WPF unloads the parent TabItem
+           without firing MouseLeave, so IsOpen stays true with a stale anchor.
+           When the chart becomes visible again, OnMouseMove sets IsOpen = true
+           but it is already true, so the popup never re-anchors and never shows.
+           Force-close on every visibility/load transition so the next mouse move
+           re-opens cleanly. */
+        chart.IsVisibleChanged += OnChartVisibilityChanged;
+        chart.Unloaded += OnChartUnloaded;
+        chart.Loaded += OnChartLoaded;
     }
 
     public string Unit { get => _unit; set => _unit = value; }
@@ -61,11 +71,23 @@ public void Dispose()
     {
         _chart.MouseMove -= OnMouseMove;
         _chart.MouseLeave -= OnMouseLeave;
+        _chart.IsVisibleChanged -= OnChartVisibilityChanged;
+        _chart.Unloaded -= OnChartUnloaded;
+        _chart.Loaded -= OnChartLoaded;
         _popup.IsOpen = false;
         _scatters.Clear();
         _barPlots.Clear();
     }
 
+    private void OnChartVisibilityChanged(object sender, DependencyPropertyChangedEventArgs e) =>
+        _popup.IsOpen = false;
+
+    private void OnChartUnloaded(object sender, RoutedEventArgs e) =>
+        _popup.IsOpen = false;
+
+    private void OnChartLoaded(object sender, RoutedEventArgs e) =>
+        _popup.IsOpen = false;
+
     public void Clear()
     {
         _scatters.Clear();
@@ -207,6 +229,11 @@ pick the series closest in Y (nearest line to cursor). */
                 _text.Text = $"{bestLabel}\n{valueFormatted} {_unit}\n{time:HH:mm:ss}";
                 _popup.HorizontalOffset = pos.X + 15;
                 _popup.VerticalOffset = pos.Y + 15;
+                /* Toggle if already open so WPF re-evaluates the placement target.
+                   Without this, a popup that was IsOpen = true when its TabItem was
+                   unloaded stays "open" with a stale anchor and never appears on
+                   return — the assignment below is a no-op. */
+                if (_popup.IsOpen) _popup.IsOpen = false;
                 _popup.IsOpen = true;
             }
             else