How to recover tabOrder after switching `windowFlags` of some widgets in Qt?

The short answer: basically you can't fix a widgets tab order automagically. Setting a QWidget as popup removes it from the QWidget tab order hierarchy because a popup is kind of a window with its very own internal tab hierarchy. By resetting your popup back to a simple widget it is then readded to the tab hierarchy at the and of the level you added it to scrambling everything inbetween. This seems to be a problem of the Qt library. I guess your idea is the only way to recover the taborder afterwards. Related thread about readding widgets. (Your Attempt 2 is basically what is suggested there)

You can verify how messed up your tab order becomes by calling this function after you tried to fix it with your very sensible Attempt 1 inside the main-function-lambda:

void printTabOrder(QWidget *start)
{
    QWidget *current = start;

    do
    {
        qDebug() << current;
        current = current->nextInFocusChain();
    } while(current != start);
}

I would advice you not to change the widgets window flags on the fly. Insted just create a customly behaving and styled QWidget as a popup. IMHO it would be better style anyway because your TestWidget is just the thing holding the buttons and handling clicks on them and your TestWidgetMenu could handle all the popup stuff.

I attribute all structural and style issues to the minimal-working-exaple-nature of this code.

1. Never use auto

2. Only use Lambda's at gun point

   auto tryRecoverThisMess = [&window, &t1, &t2, &t3, &t4]()
   {
       qDebug() << "Trying to recover this mess";
   
       // Attempt 1: not working
       window.setTabOrder( &t1, &t2 );
       window.setTabOrder( &t2, &t3 );
       window.setTabOrder( &t3, &t4 );
   

   #if 0 // Attempt 2: very hacky and not working QWidget *w=nullptr; w = t1.orderTabs( w ); w = t2.orderTabs( w ); w = t3.orderTabs( w ); w = t4.orderTabs( w ); #endif

   };
   

Had you created a class MyClass derived from QWidget and given sad class a slot void tryRecoverThisMess();

You would not have needed parameters because TestWidget items would have been member variables. More importantly the slot would only need.

setTabOrder( &t1, &t2 );
setTabOrder( &t2, &t3 );
setTabOrder( &t3, &t4 );

because it would be executing within the widget.

You are calling QTabWidget::setTabOrder inside a Lambda with an auto type. The compiler was free to either determine this Lambda was a QWidget and set the tab order for it, or to manufacture a temporary QWidget setting its tab order before disposing of it at the end of the Lambda's life.

There was nothing in your code identifying that you wanted to change the tab order of "window."

Admittedly I tried this on Ubuntu 20.04 LTS using Qt 5.12.8, but I did see things jump to the third button after the click. After my fix they didn't do that. The "correct" fix would be to put this stuff in a class and connect the SIGNAL to a slot in the class.

I suspect you have similar parenting issues in your other code.

Restructure you code to delete all Lambda's and remove all autos.

Lambda's as slots are really dangerous. They get extremely dangerous when it comes to objects created then moved to another thread. The Lambda isn't part of the object, it didn't have to move. That means you could have a queued connection and the object that this Lambda slot is trying to return to could be deleted.

Young developers like to use auto and Lambda because it lets them take shortcuts.

The longest distance between any two points is a shortcut.

When you get older you will realize that's a fact in IT.

Welcome to the wonderful world of programming!

Per your request I "patched" your code. It needs to be redesigned from scratch to be "fixed."

#include <QApplication>
#include <QDebug>
#include <QWidget>
#include <QPushButton>

class TestWidget: public QWidget
{
    Q_OBJECT
public:
    explicit TestWidget( QString txt, QWidget *parent=nullptr )
        : QWidget( parent )
        , button1( nullptr )
        , button2( nullptr )
    {
        // don't create widgets on the stack
        //
        QString lbl1 = QString( "%1-%2" ).arg( txt ).arg( 1 );
        QString lbl2 = QString( "%1-%2" ).arg( txt ).arg( 2 );
        button1 = new QPushButton( lbl1, this );
        button2 = new QPushButton( lbl2, this );

        button1->setGeometry( 10, 10, 80, 30 );
        button2->setGeometry( 110, 10, 80, 30 );

        setGeometry( 0,0,200,50 );

        resize( 200,50 );  // ????

        setStyleSheet( "QPushButton{ background-color: rgba(170, 176, 81, 200); }"
                       "QPushButton:focus{ background-color: rgba(72, 75, 224, 200); } "
                       "QPushButton:focus:pressed{ background-color: rgba(229, 141, 29, 200); } "
                       "QPushButton:pressed{ background-color: rgba(241, 5, 10, 250); } " );

        setFocusProxy( this );
        setFocusProxy( button1 );
        setFocusPolicy( Qt::FocusPolicy::StrongFocus );

        setInternalTabOrder();
        connect( button1, &QPushButton::clicked, [this]()
        {
            const auto flag = this->windowFlags();

            // In some circumstances, the widget needs to show it content as drop-down
            this->setWindowFlags( Qt::Popup | Qt::FramelessWindowHint );

            //try to recover here
            this->setWindowFlags( flag );
            this->show();
            emit needsTabOrder();
            this->setFocus( );  // without this clicked button won't grab
            // focus but a clicked button2 will
        } );
    }

    ~TestWidget()
    {
        if ( button1 != nullptr )
        {
            delete button1;
            button1 = nullptr;
        }

        if ( button2 != nullptr )
        {
            delete button2;
            button2 = nullptr;
        }
    }

    void setInternalTabOrder()
    {
        setTabOrder( button1, button2 );
    }
signals:
    void needsTabOrder();
private:

    QPushButton *button1;
    QPushButton *button2;
};
#include "main.moc"


auto main ( int argn, char *args[] )-> int
{
    QApplication app( argn, args );

    // don't create widgets on the stack
    //
    QWidget *window = new QWidget();

    TestWidget *t1 = new TestWidget( "Fred", window );
    TestWidget *t3 = new TestWidget( "Ethyl", window ); //Intentionally out of order
    TestWidget *t2 = new TestWidget( "Lucy", window );
    TestWidget *t4 = new TestWidget( "Ricky", window );

    t1->move( 0,10 );
    t2->move( 200,10 );
    t3->move( 400,10 );
    t4->move( 600,10 );

    QWidget::setTabOrder( t1, t2 );
    QWidget::setTabOrder( t2, t3 );
    QWidget::setTabOrder( t3, t4 );

    auto tryRecoverThisMess = [window, t1, t2, t3, t4]()
    {
        qDebug() << "Trying to recover this mess";

        window->setTabOrder( t1, t2 );
        window->setTabOrder( t2, t3 );
        window->setTabOrder( t3, t4 );

    };

    QWidget::connect( t1, &TestWidget::needsTabOrder, tryRecoverThisMess );
    QWidget::connect( t2, &TestWidget::needsTabOrder, tryRecoverThisMess );
    QWidget::connect( t3, &TestWidget::needsTabOrder, tryRecoverThisMess );
    QWidget::connect( t4, &TestWidget::needsTabOrder, tryRecoverThisMess );

    window->resize( 800,200 );
    window->show();

    return app.exec();
}

Despite all of the down votes from haters who want to use auto and lambda everywhere, this is the only working code that has been posted, and it has been tested. Nobody else took the time to actually write working code.