Custom Validator does not work if text is selected from history

Hi everybody,
In a text box when I select text from the history list of that text box and press TAB, the custom validator does not fire.
However, if I enter the same text manually and press TAB it fires. I want the validator to fire even if the text is selected from the history list. Is that possible? Or is there a way to disable the history from coming up?

I suppose that happens because form field values are being stored for <input> types of a certain specific ID.

And I also think it's a browser specific feature. The best you could do would be to use the onChange event of the textbox, but that would also validate for every keystroke, which is quite annoying.