1. Switch all Qt-based components (FSL and gmex) to QT6.

2. Do not query SoQt for line width limits but use fixed values.  The
   query function is not documented and no longer works.  This also
   results in a simplification of the gmex code.

CMakeLists.txt

-# Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration
-
-# === Preamble ===
-cmake_minimum_required(VERSION 3.16...4.0)
-
-# === Project's settings ===
-include( ${CMAKE_CURRENT_SOURCE_DIR}/cmake/GeoModel-version.cmake )
-project( "GeoModel" VERSION ${GeoModel_VERSION} LANGUAGES CXX )
-
-set(CMAKE_CXX_STANDARD 20)
-
-# Make the 'cmake' module directory visible to CMake.
-list( APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake )
-
-# === Project wide setup ===
-# Define color codes for CMake messages
-include( cmake_colors_defs )
-# Use the GNU install directory names.
-include( GNUInstallDirs )
-# Set a default build type
-include( BuildType )
-# Set default build and C++ options
-include( configure_cpp_options )
-
-# By default prefer not to use frameworks on macOS.
-# But use it at a "LAST" resource, if no other options worked.
-# For details on the behaviour of this cache variable, see:
-# - https://cmake.org/cmake/help/v3.0/command/find_file.html
-if( APPLE )
-  set( CMAKE_FIND_FRAMEWORK "LAST" CACHE STRING "Framework finding behaviour on macOS" )
-endif()
-
-# === Externally provided content ===
-# Set up how the project handle some of its dependenices. Either by picking them
-# up from the environment, or building them itself.
-include( SetupEigen3 )
-include( SetupXercesC )
-include( SetupJSON )
-
-# Find the dependencies that the project always picks up from its environment.
-find_package( SQLite3 3.7.17 )
-
-# === Main targets built by this project ===
-
-# switches to let users build specific packages on request
-option(GEOMODEL_BUILD_ALL "Enable the build of all GeoModel sub-packages" OFF)
-option(GEOMODEL_BUILD_TOOLS "Enable the build of GeoModelTools" OFF)
-option(GEOMODEL_BUILD_EXAMPLES "Enable the build of GeoModelExamples" OFF)
-option(GEOMODEL_BUILD_EXAMPLES_W_GEANT4 "Enable the build of GeoModelExamples, including the ones concerning the interface GeoModel->Geant4" OFF)
-option(GEOMODEL_BUILD_VISUALIZATION "Enable the build of GeoModelVisualization" OFF)
-option(GEOMODEL_BUILD_GEOMODELG4 "Enable the build of GeoModelG4" OFF)
-option(GEOMODEL_BUILD_FULLSIMLIGHT "Enable the build of FullSimLight" OFF)
-option(GEOMODEL_BUILD_FULLSIMLIGHT_PROFILING "Enable FullSimLight profiling targets" OFF)
-option(GEOMODEL_BUILD_FSL "Enable the build of FSL and FullSimLight" OFF)
-option(GEOMODEL_BUILD_ATLASEXTENSIONS "Build the Custom ATLAS Extensions" OFF)
-option(GEOMODEL_BUILD_TESTING "Build the unit tests of GeoModel" OFF )
-option(GEOMODEL_USE_QT6 "Use QT6 for Visualization and FSL" ON)
-
-include(CTest) # needs to be included at the top level
-if( BUILD_TESTING AND GEOMODEL_BUILD_TESTING )
-    # Add the GoogleTest framework, for tests
-    find_package(GTest REQUIRED)
-    include_directories(${GTEST_INCLUDE_DIRS})
-endif()
-
-
-if(GEOMODEL_BUILD_FSL AND GEOMODEL_BUILD_FULLSIMLIGHT)
-  message(STATUS "==> NOTE: You enabled both BUILD_FSL and BUILD_FULLSIMLIGHT. Since BUILD_FSL triggers the build of the FullSimLight package as well, I set BUILD_FULLSIMLIGHT to FALSE to avoid errors of 'double inclusion'")
-  set(GEOMODEL_BUILD_FULLSIMLIGHT FALSE)
-endif()
-
-if (GEOMODEL_USE_QT6)
-  set (QT_VERSION 6)
-else()
-  set (QT_VERSION 5)
-endif()
-
-if(GEOMODEL_BUILD_FULLSIMLIGHT_PROFILING)
-  set(GEOMODEL_BUILD_FULLSIMLIGHT ON CACHE BOOL "Enable the build of FullSimLight" FORCE)
-endif()
-
-# a list to keep track of the packages we build
-set(BUILT_PACKAGES "")
-
-add_subdirectory(GeoModelCore)
-list( APPEND BUILT_PACKAGES "GeoModelCore")
-add_subdirectory(GeoModelIO)
-list( APPEND BUILT_PACKAGES "GeoModelIO")
-
-if(GEOMODEL_BUILD_ALL)
-  set(GEOMODEL_BUILD_TOOLS         TRUE)
-  set(GEOMODEL_BUILD_VISUALIZATION TRUE)
-  set(GEOMODEL_BUILD_FSL           TRUE)
-  set(GEOMODEL_BUILD_TESTING       TRUE)
-endif()
-
-if(GEOMODEL_BUILD_TOOLS)
-  add_subdirectory(GeoModelTools)
-  list( APPEND BUILT_PACKAGES "GeoModelTools")
-endif()
-
-if(GEOMODEL_BUILD_VISUALIZATION)
-  add_subdirectory(GeoModelVisualization)
-  list( APPEND BUILT_PACKAGES "GeoModelVisualization")
-endif()
-
-if(GEOMODEL_BUILD_EXAMPLES)
-  add_subdirectory(GeoModelExamples)
-  list( APPEND BUILT_PACKAGES "GeoModelExamples")
-endif()
-
-if(GEOMODEL_BUILD_FULLSIMLIGHT)
-  set(GEOMODEL_BUILD_GEOMODELG4 TRUE) # FullSimLight needs GeoModelG4
-  add_subdirectory(FullSimLight)
-  list( APPEND BUILT_PACKAGES "FullSimLight")
-endif()
-
-if(GEOMODEL_BUILD_FSL)
-  set(GEOMODEL_BUILD_GEOMODELG4 TRUE) # FullSimLight needs GeoModelG4
-  add_subdirectory(FullSimLight)# FSL needs FullSimLight
-  add_subdirectory(FSL)
-  list( APPEND BUILT_PACKAGES "FullSimLight")
-  list( APPEND BUILT_PACKAGES "FSL")
-endif()
-
-if(GEOMODEL_BUILD_ATLASEXTENSIONS)
-    if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/ATLASExtensions)
-        if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/ATLASExtensions/CMakeLists.txt)
-            add_subdirectory(ATLASExtensions)
-            list( APPEND BUILT_PACKAGES "ATLASExtensions")
-        else()
-            message(FATAL_ERROR "You enabled the build of the ATLASExtensions submodule, by setting the 'GEOMODEL_BUILD_ATLASEXTENSIONS=1' option. However, no 'CMakeLists.txt' file was found in the ATLASExtensions folder! You probably forgot to recursively clone the submodule. Run this from the source top folder: 'git submodule update --init'. Then, re-run CMake." )
-        endif()
-    else()
-        message(FATAL_ERROR "You enabled the build of the ATLASExtensions submodule, by setting the 'GEOMODEL_BUILD_ATLASEXTENSIONS=1' option. However, no 'ATLASExtensions' folder is found in the repository! Please, check the integrity of your copy of the GeoModel repository." )
-    endif()
-endif()
-
-
-if(GEOMODEL_BUILD_GEOMODELG4 OR GEOMODEL_BUILD_EXAMPLES_W_GEANT4)
-  add_subdirectory(GeoModelG4)
-  if(GEOMODEL_BUILD_EXAMPLES AND GEOMODEL_BUILD_EXAMPLES_W_GEANT4)
-  message(STATUS "==> NOTE: You enabled both GEOMODEL_BUILD_EXAMPLES and GEOMODEL_BUILD_EXAMPLES_W_GEANT4. Since GEOMODEL_BUILD_EXAMPLES triggers the build of the GEOMODEL_BUILD_EXAMPLES_W_GEANT4 as well, I set GEOMODEL_BUILD_EXAMPLES_W_GEANT4 to FALSE to avoid errors of 'double inclusion'")
-  set(GEOMODEL_BUILD_EXAMPLES_W_GEANT4 FALSE)
-  elseif (GEOMODEL_BUILD_EXAMPLES_W_GEANT4)
-     add_subdirectory(GeoModelExamples)
-     list( APPEND BUILT_PACKAGES "GeoModelExamples")
-  endif()
-  list( APPEND BUILT_PACKAGES "GeoModelG4")
-endif()
-
-# Print Build Info on screen
-include( PrintBuildInfo )
-
-# Let the users know which and how many packages they are building
-list(LENGTH BUILT_PACKAGES BUILT_PACKAGES_LENGTH)
-string(REPLACE ";" ", " BUILT_PACKAGES_STR "${BUILT_PACKAGES}")
-
-message(STATUS "${BoldWhite}-----${ColourReset}")
-message(STATUS "${BoldGreen}Building the following ${BUILT_PACKAGES_LENGTH} packages: ${BUILT_PACKAGES_STR}${ColourReset}")
-message(STATUS "${BoldWhite}-----${ColourReset}")