Fix issue.

Author: Yuncong Zhang

Runtime/rendering/editable.cs

@@ -668,6 +668,10 @@ public TextSelection selection {
                 return this._selection;
             }
             set {
+                if (this._selection == value) {	
+                    return;	
+                }
+                
                 this._selection = value;
                 this._selectionRects = null;
                 this.markNeedsPaint();