Create a popup window in Qt without a shadow

I worked out a solution to the question I posted earlier.

First, a popup widget can be created by deriving from QWidget with flags Qt::Window | Qt::FramelessWindowHint. I actually hacked qwidget_win.cpp to ensure my window has the same style and extended style as the Popup control provided by WPF (0x96000000 and 0x08000088 respectively), as determined using Spy++, however I don't think that should matter.

Setting the window style doesn't solve the Window activation problem. The key to doing this to intercept the windows messages that inform the main window that it's non-client area should be updated (WM_NCACTIVATE message). This can be done by providing a custom implementation of QApplication::winEventFilter(MSG* msg, long* result).

Unfortunately, simply eating inactivate WM_ NCACTIVATE messages isn't enough, because it seems to prevent WM_ACTIVATE and other messages that otherwise get generated from being sent to the popup window. To solve this, I have a proof of concept working where I generate the required windows messages in winEventFilter after having caught an appropriate WM_NCACTIVATE message.

From here, there are some details to work through to make sure it's robust (intercept WM_ACTIVATEAPP comes to mind), and then wrap it up into something nice and reusable.

All this is windows specific of course, be interesting to see if there are issues under Linux.

All this is a bit of a pain. Hope I didn't miss something obvious...

Try this strange code bellow (Tested on Qt v5.*):

//BasePopup.h
#ifndef BASEPOPUP_H
#define BASEPOPUP_H
#include <QFrame>

class BasePopup : public QFrame
{
    Q_OBJECT
public:
    explicit BasePopup(QWidget* parent = nullptr);
};

#endif // BASEPOPUP_H

//BasePopup.cpp
#include "BasePopup.h"

BasePopup::BasePopup(QWidget *parent)
    : QFrame(parent, Qt::Window | Qt::FramelessWindowHint)
{
    /*setAttribute(Qt::WA_TranslucentBackground);*/ /* if need transparent background; 
    set this attribute in another place create window with black background */
    show();       // must be, if not system shadow occur :) I don't know why
    setWindowFlags(Qt::Popup | Qt::FramelessWindowHint);
    show();
    //---------------------- 
    //yours code here 
    //----------------------
}

Works only with this sequence methods call. If make something change, we had popup with shadow. ( Magic :))) )

Edit

I find this flag Qt::NoDropShadowWindowHint in enum Qt::WindowFlags (Qt v5.3). Which appeared in one version of Qt v5.*. With this flag above code must look something like this.

//BasePopup.cpp
#include "BasePopup.h"

BasePopup::BasePopup(QWidget *parent)
    : QFrame(parent, Qt::Popup | Qt::FramelessWindowHint | Qt::NoDropShadowWindowHint)
{
    //---------------------- 
    //yours code here 
    //----------------------
}

I not tested yet, but i think it must work.

I am thinking the shadow that you see is related to the Windows theme that you are using. I am using the Windows Classic Theme here and i don't see any shadows :)

Anyways, what if you connect the textChanged() SIGNAL of the QLineEdit into a custom action that restores the focus of the Main Window? Something like:

void PopupTest::handleClick()
{
   QFrame* popup1 = new QFrame(this, Qt::Tool | Qt::Window | Qt::FramelessWindowHint);
   popup1->resize(150,100);
   QLineEdit *tmpE = new QLineEdit( popup1 );
   connect( tmpE, SIGNAL( returnPressed() ), popup1, SLOT( hide() ) );
   connect( tmpE, SIGNAL( textChanged(const QString&)), this, SLOT( setActive() ) );
   tmpE->setGeometry(10,10, 130, 30);
   tmpE->setFocus();
   popup1->show();
}

void MainWindow::setActive()
{
   this->activateWindow();
}

You will have to create a slot called setActive() and you should also put the QLineEdit into your class header so that from the setActive() function you can do something like:

tmpE->setFocus();

to restore the focus on the QLineEdit as well.