@jdent U:\Users\Juan Dent\Qt\with QtCreator\build-QtQuickApp-Desktop_Qt_6_6_2_MSVC2019_64bit-Debug_deps is a bit long.

Have a look at https://bugreports.qt.io/browse/QTBUG-117413
and if you are building a Qt Design Studio project change the corresponding CMake code like this:

if (BUILD_QDS_COMPONENTS) if (WIN32) set(FETCHCONTENT_BASE_DIR $ENV{HOMEDRIVE}/bld/qds-deps) # <---- new line endif() include(${CMAKE_CURRENT_SOURCE_DIR}/qmlcomponents) endif()

TL;DR; Add #include "ui_Task.h" in your Task.cpp file.

Longer explanation - the uic tool takes the Task.ui file and generates ui_Task.h header, which contains the definition for the Ui::Task class. This header is to be included in your Task.cpp file so that you can use it to initialize your ui by first creating an instance of that class (it's the new Ui::Task line that gives you the error about incomplete type) and then calling ui->setupUi(this) on it.

Compare that with the wizard generated code - see the #include "ui_Mainwindow.h" line in the Mainwindow.cpp file.

Okay, I managed to figure it out. Yes, this module is not available if using open source version of Qt but one can build it manually from source components and use.

This instruction helped me a lot: https://linzichun.com/posts/qt-mqtt-opcua-modbus-rest/#qt-opc-ua

Be ready to read the console, download some other missing stuff to your platform and repeat the same command, however. I needed to install about 4-5 missing utilities during the proccess.

New discovery, if it is helpful to future readers:

Changing the font size/type causes a change.

In my case, I had Kawkab Mono size 10, which is bigger than the default KDE font, Noto Sans, same size, because the latter does not reproduce the issue.

I lowered the size of my font, and 7 is the highest value I could use to get a small context menu, and that is using a 1920x1080 resolution, 100% scale.

To confirm, I changed the resolution to one that is close to what I had on my old laptop, 1368x768, and I was able to get a small context menu with 4 as the system's font size, Kawkab Mono again (because it is the best I could find, that keeps Arabic letters mono as well in Qt Creator's text editor).

So, lower the system font size, if your eyes can tolerate that. Or just use shortcuts.