Qt6 platform rendering issues on Ubuntu 22.04

[Robert Hairgrove]

After updating my C++ desktop application to build with Qt 6.5, I noticed strange changes in the rendering of the widgets. For example, there are no window borders, and setting enabled = false for labels, etc. does not make them appear disabled. I have to add a command-line option -platform xcb to run my app in order to see the title bar.

Here are the screenshots of the different ways the main window is rendered for comparison:

1. Built with Qt 5.15.5:
   

2. Built with Qt 6.5, default appearance (no command-line option) ... notice how there is no window border:
   

3. Built with Qt 6.5, command-line option -platform xcb added:
   

Notice that on Qt 5, my "To do" list is nicely set to disabled gray look, whereas all labels look enabled with Qt 6 in both renderings.

What is the least intrusive way of making the newer versions look like the older ones?

[Robert Hairgrove]

Here is a screenshot with an active database in the app built with Qt 5.15, showing the "To do" list with different stages enabled and disabled:

[SGaist]

Hi,

Do you have the same issue if you use the fusion style ?

By the way, which desktop environment are you using ?

[Robert Hairgrove]

@SGaist Thanks for looking at this. I am basically running a default desktop which Ubuntu 22.04 has configured. I don't really know what I am doing here, since I never changed anything. But I saw somewhere references to environment variables beginning with XDG_..., so here are the results of a few of those:

bob@bobs-laptop:~$ echo $XDG_SESSION_TYPE 
wayland
bob@bobs-laptop:~$ echo $XDG_SESSION_DESKTOP 
ubuntu
bob@bobs-laptop:~$ echo $XDG_CURRENT_DESKTOP 
ubuntu:GNOME

If I give this option on the command line: -patform xcb as mentioned above, are there any options I can add after that? I looked all over but could not find any documentation about what the allowed options are, except for -platform windows which has a lot of extra options.

As to fusion, I get a fatal error when I put ? -platform fusion` on the command line:

This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: xcb, offscreen, minimal, vkkhrdisplay, eglfs, vnc, linuxfb, wayland, minimalegl, wayland-egl.

What do I need to do in order to get fusion to work?

[Robert Hairgrove]

@SGaist I found out how to set the "Fusion" style here in another forum thread.

So I added this line to my main() function right after creating the instance of QApplication:

qApp->setStyle(QStyleFactory::create("Fusion"));

It doesn't have any effect, AFAICT.

[JoeCFD]

@Robert-Hairgrove I just wrote a small test program to confirm that qlabel is greyed out if it is disabled. Qt: 6.5.2 and OS: Ubuntu 22.04.

[Robert Hairgrove]

@JoeCFD Thanks ... would you mind sharing your code with us? :)

[JoeCFD]

@Robert-Hairgrove
main.cpp

#include "helpwidget.h"

#include <QApplication>

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

    auto widget = new HelpWidget;
    widget->show();

    return app.exec();
}

helpwidget.h

#ifndef HELPWIDGET_H
#define HELPWIDGET_H

#include <QWidget>

//! [0]
class HelpWidget : public QWidget
{
    Q_OBJECT

public:
    HelpWidget(QWidget *parent = nullptr);

};
//! [0]

#endif // HelpWidget_H

helpwidget.cpp

#include <QVBoxLayout>
#include <QLabel>
#include <QPushButton>


#include "helpwidget.h"

HelpWidget::HelpWidget( QWidget * parent )
    : QWidget(parent) 
{    
    auto layout = new QVBoxLayout( this );
    setMinimumSize( 400, 400 );

    auto label = new QLabel( "Dis/enabled test", this );
    auto button = new QPushButton( "Press", this );

    layout->addWidget( label );
    layout->addWidget( button );

    connect( button, &QPushButton::pressed,
             [=](){  auto isEnabled = label->isEnabled();
                     label->setEnabled( !isEnabled ); } );
}

[Robert Hairgrove]

@JoeCFD Thanks!

Could you also please post the contents of the *.pro file (or CMake project file) that you used to build this?

[JoeCFD]

@Robert-Hairgrove There you.
testlabel.pro

QT += widgets

HEADERS += \
        helpwidget.h 

SOURCES += \
        helpwidget.cpp \
        main.cpp

[Robert Hairgrove]

@JoeCFD Your code builds and runs OK, but when I click on the "Press" button, the label does not change its appearance.

Please post the results of the following commands run in a terminal:

echo $XDG_SESSION_TYPE
echo $XDG_SESSION_DESKTOP
echo $XDG_CURRENT_DESKTOP

Also:

echo $QT_QPA_PLATFORM

Thank you!

[JoeCFD]

@Robert-Hairgrove
x11
Lubuntu
LXQt
empty

[JonB]

@Robert-Hairgrove
@SGaist understands this much better than I do, but is your picture #2 (no borders and Qt6.5) somehow "wayland"? Which might not have/do the effects of x11/xcb?

[Robert Hairgrove]

@JonB Yes, it is wayland if I do not set "-platform xcb" as a command line option.

[Robert Hairgrove]

I forgot to add that the precise version of Qt I have is 6.5.3 which was installed automatically by the Qt Creator maintenance tool.

[JoeCFD]

@Robert-Hairgrove I guess the problem should be gone if you switch to X11. Do you have to use Wayland? Wayland is the default setting in Ubuntu 22.04.

[Robert Hairgrove]

@JoeCFD "X11" is not listed as an available platform argument (see my reply to @SGaist above).

How would I do this, switch to X11?

[JoeCFD]

@Robert-Hairgrove logout your account. You can see a settings icon on the login screen and click the setting icon to select Xorg. Then log in and you will be good.

[Robert Hairgrove]

@JoeCFD Thanks.

I logged in again with XOrg, but unfortunately the problem persists.

[Robert Hairgrove]

@JoeCFD BTW, what is "LXQt"?