On OS X 10.10 the checkbox becomes inactive while parent widget haven't focus.

STEPS TO REPRODUCE
(Qt Creator 3.3.0 Based On Qt 5.4.0 (clang 6.0(Apple), 64 bit))

1. Create simple project for Dialog window.
2. Add to the *.ui file QLineEdit, QListWidjet and layout without any modifications.
3. Code to add list items in the constructor of the dialog window:

QListWidget* list = ui->listWidget;
for( int i = 0; i < 5; ++i )
{
QListWidgetItem* item = new QListWidgetItem( "test" );
item->setCheckState( Qt::Checked );

list->addItem( item );
}

Snapshots are added.