Disable selection for all buttons.

disable-selection-button.css

button {
    user-select:none;          /* Actual property.                  */
    /* For compatibility with older browsers                        */
    -o-user-select:none;       /* Opera browser support             */
    -ms-user-select:none;      /* IE and Microsoft Edge support     */
    -moz-user-select:none;     /* Firefox support                   */
    -webkit-user-select:none;  /* Webkit engine (Chrome and Safari) */
}