Fix #4340 (#4491)

Co-authored-by: Denis Voituron <dvoituron@outlook.com>

Author: vnbaaij

examples/Demo/Shared/Pages/DataGrid/Examples/DataGridMultiSelect.razor

@@ -53,7 +53,7 @@ else
     </FluentDataGrid>
 
     <div>
-        <b>Peoples:</b>
+        <b>People:</b>
         @String.Join("; ", People.Where(p => p.Selected).Select(p => p.Name))
     </div>
 }

src/Core/Components/DataGrid/Columns/SelectColumn.cs

@@ -183,7 +183,7 @@ public DataGridSelectMode SelectMode
     /// Null is undefined.
     /// </summary>
     [Parameter]
-    public bool? SelectAll { get; set; } = false;
+    public bool? SelectAll { get; set; }
 
     /// <summary>
     /// Gets or sets the action to be executed when the [All] checkbox is clicked.
@@ -521,10 +521,9 @@ private void RefreshHeaderContent()
     /// <summary />
     private bool? GetSelectAll()
     {
-        // Using SelectedItems only
         if (InternalGridContext != null && (Grid.Items != null || Grid.ItemsProvider != null))
         {
-            if (_selectedItems.Count == 0)
+            if (_selectedItems.Count == 0 && !InternalGridContext.Items.Any(Property))
             {
                 return false;
             }