Ubuntu – Selected Text missing when press ctrl or alt


I am a Ubuntu 14.04 LTS user. I encounter a problem when I type in any text box of browser.

When I select any text in the text box of a browser (Chrome or FireFox)
, once I press ctrl or alt. The selected text is deleted. But it does not happen in any place else such as (LibreOffice, text editor, or terminal).
Because it happens in chrome and FireFox both, I guess it is not the problem of the browser.
Does anyone have any idea about it?