QLineEdit does not properly handle trailing actions when there is the clear button

The fix for bug 39660 (which landed on Qt 5.5.1) was (also) supposed to correctly hide a trailing hidden QAction in a QLineEdit. However, I've just upgraded my system to Qt 5.5.1 and the issue is only partially fixed.

Steps to reproduce:

1. Open a KDE application which uses a KPasswordDialog (with KF5 >= 5.15), e.g. Ark with a password-protected zip archive.
2. Make the application open the KPasswordDialog
3. There is a greyed-out icon with eye shape (let's call it "visibility action") -> Expected result: there should be no icon, since the visibility action is initially hidden.
4. Type a password. The visibility action is now visible, but so is the clearButton action. The QLineEdit displays the visibilty action on top of the clearButton action, which cannot be triggered. -> Expected result: the visibility action should be displayed on the left of the clearButton action.
5. Delete the password typed. Both the actions disappear, and this is fine.
6. Type again a password. Now the visibility action is shown on the left of the clearButton action, as expected.

So, the issue is about steps 3 and 4.

UPDATE:
Step 4 is probably a duplicate of QTBUG-48806

For step 3, the following is a minimal code example to reproduce the issue:

#include <QAction>
#include <QApplication>
#include <QIcon>
#include <QLineEdit>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QLineEdit lineEdit;
    QPixmap pixmap(16, 16);

    pixmap.fill(Qt::red);
    QAction *action = lineEdit.addAction(QIcon(pixmap), QLineEdit::TrailingPosition);
    action->setVisible(false);

    lineEdit.setText("Hidden action is greyed-out.");
    lineEdit.show();

    return a.exec();
}

In the attached screenshot, you can see that the trailing action looks like disabled, while should not be visible at all.