multiple categories selection method

Dear all,
The current flexicontent method to select multiple categories is to press the ctrl or Shift key and use mouse to click the categories, but sometimes we might accidentally click a category without pressing the ctrl or Shift key, then all the previous selected categories are reset/clear (it can be more than 10 categories selected).
Although we can load back the previous version, but it is really inconvenient when we already edit the content.
I suggest that the selected categories will remain highlighted forever until we exactly click the same categories again to clear them.
Thanks!