How to switch focus without losing preedit

Isopod

Hi. Let's say I have the following requirements:

a widget (e.g. treeview)
a QLineEdit
when the treeview is focused and a key is typed, the QLineEdit is automatically focused and all input is redirected to it

It is easy to achieve this for western input by doing something like this:

bool MainWindow::eventFilter(QObject *obj, QEvent *event)
{
	QApplication *app = static_cast<QApplication*>(QApplication::instance());

	if (obj == ui->treeview && event->type() == QEvent::KeyPress) {
		ui->lineEdit->setFocus();
		app->sendEvent(ui->lineEdit, event);
		return true;
	}
	return false;
}

The problem is, this doesn't work when an IME is used, since no QEvent::KeyPress events are generated in that case. I've tried forwarding QEvent::InputMethod in addition to QEvent::KeyPress, but that leads to more problems:

QEvent::InputMethod is emitted as soon as a widget receives focus, even if nothing has been typed yet.
As a workaround, I could only redirect if the preedit is not empty, but then the IME is reset by ui->lineEdit->setFocus and the preedit is lost

How can I set focus to a widget without resetting the IME in Qt5? I thought this would be a common usecase, I'm surprised it is so difficult.