fix(ui): show the newly selected plate when a Plates row is chosen

The plate preview read DataGridView.CurrentRow in SelectionChanged, but
that event runs before CurrentCell moves, so CurrentRow still named the
previous row. Choosing a row showed the plate that had been selected
before it, and the next-plate button snapped the preview back to the
old plate. Read the selected row instead.

The Windows job of run 37341610182 caught it:
PlatePreview_StepsThroughThePlates_AndFollowsThePlatesTable expected
"Plate 2 of 2" after the next-plate button and got "Plate 1 of 2". The
test now also checks that the selected row follows the button.

OpenNest.WinForms.Tests/Forms/SavedNestsFormTests.cs

@@ -85,6 +85,7 @@ public class SavedNestsFormTests
                 Assert.Equal("Plate 2 of 2", label.Text);
                 Assert.Single(preview.Plate.Parts);
                 Assert.Equal(1, plates.CurrentRow!.Index);
+                Assert.Equal(1, plates.SelectedRows[0].Index);
                 Assert.True(previous.Enabled);
                 Assert.False(next.Enabled);
 

OpenNest/Forms/SavedNestsForm.cs

@@ -139,8 +139,10 @@ public sealed class SavedNestsForm : Form
         AddColumn(platesGrid, "Utilization", "Utilization", 90, typeof(double), alignRight: true, format: "P1");
         platesGrid.SelectionChanged += (_, _) =>
         {
-            if (platesGrid.CurrentRow is { } row)
-                ShowPlate(row.Index);
+            // Read the selected row: while this event runs, CurrentRow still names the
+            // previously current row.
+            if (SelectedPlateRow() is var row and >= 0)
+                ShowPlate(row);
         };
 
         drawingsGrid = CreateGrid();
@@ -641,10 +643,12 @@ public sealed class SavedNestsForm : Form
         plateLabel.Text = $"Plate {index + 1:N0} of {previewPlates.Count:N0}";
         previousPlateButton.Enabled = index > 0;
         nextPlateButton.Enabled = index < previewPlates.Count - 1;
-        if (index < platesGrid.Rows.Count && platesGrid.CurrentRow?.Index != index)
+        if (index < platesGrid.Rows.Count && SelectedPlateRow() != index)
             platesGrid.CurrentCell = platesGrid.Rows[index].Cells[0];
     }
 
+    private int SelectedPlateRow() => platesGrid.SelectedRows.Count > 0 ? platesGrid.SelectedRows[0].Index : -1;
+
     private void OpenSelected()
     {
         if (SelectedNestId() is not Guid id)