feat: fix date range start selection when tab into the control.

src/Ursa/Controls/DateTimePicker/DateRangePicker.cs

@@ -458,4 +458,31 @@ public class DateRangePicker : DatePickerBase, IClearControl
         }
     }
 
+    protected override void OnGotFocus(GotFocusEventArgs e)
+    {
+        base.OnGotFocus(e);
+        FocusChanged(IsKeyboardFocusWithin);
+    }
+
+    private bool _isFocused;
+    private void FocusChanged(bool hasFocus)
+    {
+        // The OnGotFocus & OnLostFocus are asynchronously and cannot
+        // reliably tell you that have the focus.  All they do is let you
+        // know that the focus changed sometime in the past.  To determine
+        // if you currently have the focus you need to do consult the
+        // FocusManager.
+
+        bool wasFocused = _isFocused;
+        _isFocused = hasFocus;
+
+        if (hasFocus)
+        {
+
+            if (!wasFocused && _start == null)
+            {
+                _start = true;
+            }
+        }
+    }
 }