In order to migrate to Qt6 all options for KF6 were added:

a) using a system provided KF6
b) downloading and compiling KF6
c) using the vendored-in re-creation of the functionality

The behaviour for both Qt5 and Qt6 is steered with the same two variables which were renamed to become version agnostic:
a) BUILD_WITH_KF=ON BUILD_KF=OFF
b) BUILD_WITH_KF=ON BUILD_KF=ON
c) BUILD_WITH_KF=OFF

The version is automatically derived from the chosen Qt major version.

cmake/define_definitions.cmake

@@ -62,10 +62,10 @@ message("PROJECT_SOURCE_DIR        :" ${PROJECT_SOURCE_DIR})
 message("QET_DIR                   :" ${QET_DIR})
 message("GIT_COMMIT_SHA            :" ${GIT_COMMIT_SHA})
 
-if(BUILD_WITH_KF5)
-  message("KF5_GIT_TAG               :" ${KF5_GIT_TAG})
+if(BUILD_WITH_KF)
+  message("KF_GIT_TAG               :" ${KF5_GIT_TAG})
 else()
-  add_definitions(-DBUILD_WITHOUT_KF5)
+  add_definitions(-DBUILD_WITHOUT_KF)
 endif()
 message("QET_COMPONENTS            :" ${QET_COMPONENTS})
 message("QT_VERSION_MAJOR          :" ${QT_VERSION_MAJOR})

cmake/developer_options.cmake

@@ -31,8 +31,11 @@ add_definitions(-DQT_MESSAGELOGCONTEXT)
 # In order to do so, uncomment the following line.
 #add_definitions(-DTODO_LIST)
 
-# Build with KF5
-option(BUILD_WITH_KF5 "Build with KF5" ON)
+# Build with KDE Frameworks. The major version (KF5/KF6) is derived
+# automatically from QT_VERSION_MAJOR -- KDE Frameworks deliberately
+# mirrors Qt's own major version numbering, so there is no independent
+# choice to make here. See cmake/fetch_kdeaddons.cmake.
+option(BUILD_WITH_KF "Build with KDE Frameworks" ON)
 
 # Precompiled headers for the Qt umbrella headers.
 #

cmake/fetch_kdeaddons.cmake

@@ -16,17 +16,24 @@
 
 message(" - fetch_kdeaddons")
 
-if(BUILD_WITH_KF5)
+# TODO remove path as soon as Qt5 gets retired
+if(BUILD_WITH_KF)
   Include(FetchContent)
 
-  option(BUILD_KF5 "Build KF5 libraries, use system ones otherwise" YES)
+  option(BUILD_KF "Build KF5 libraries, use system ones otherwise" YES)
 
-  if(BUILD_KF5)
+  if(BUILD_KF)
 
-    if(NOT DEFINED KF5_GIT_TAG)
+  if(KF_MAJOR_VERSION EQUAL 5)
+    if(NOT DEFINED KF_GIT_TAG)
       #https://qelectrotech.org/forum/viewtopic.php?pid=13924#p13924
-      set(KF5_GIT_TAG v5.77.0)
+      set(KF_GIT_TAG v5.77.0)
     endif()
+  else()
+    if(NOT DEFINED KF_GIT_TAG)
+      set(KF_GIT_TAG v6.10.0)
+    endif()
+  endif()
 
     # Fix stop the run autotests of kcoreaddons
     # see
@@ -38,30 +45,40 @@ if(BUILD_WITH_KF5)
     # https://qelectrotech.org/forum/viewtopic.php?pid=13929#p13929
     set(KDE_SKIP_TEST_SETTINGS "TRUE")
     set(BUILD_TESTING "0")
+    
+    # QElectroTech is a plain QtWidgets application with no QML anywhere in
+    # it; these disable optional features of the fetched KF modules that
+    # would otherwise pull in extra Qt6 components (e.g. Qt6Qml) we don't
+    # have and don't need.
+    set(BUILD_DESIGNERPLUGIN OFF)
+    set(KCOREADDONS_USE_QML OFF)
+    set(BUILD_QCH OFF)
+    set(BUILD_SHARED_LIBS OFF)
+
     FetchContent_Declare(
       ecm
       GIT_REPOSITORY https://invent.kde.org/frameworks/extra-cmake-modules.git
-      GIT_TAG        ${KF5_GIT_TAG})
+      GIT_TAG        ${KF_GIT_TAG})
     FetchContent_MakeAvailable(ecm)
 
     FetchContent_Declare(
       kcoreaddons
       GIT_REPOSITORY https://invent.kde.org/frameworks/kcoreaddons.git
-      GIT_TAG        ${KF5_GIT_TAG})
+      GIT_TAG        ${KF_GIT_TAG})
     FetchContent_MakeAvailable(kcoreaddons)
 
     FetchContent_Declare(
       kwidgetsaddons
       GIT_REPOSITORY https://invent.kde.org/frameworks/kwidgetsaddons.git
-      GIT_TAG        ${KF5_GIT_TAG})
+      GIT_TAG        ${KF_GIT_TAG})
     FetchContent_MakeAvailable(kwidgetsaddons)
   else()
-    find_package(KF5CoreAddons REQUIRED)
-    find_package(KF5WidgetsAddons REQUIRED)
+    find_package(KF${KF_MAJOR_VERSION}CoreAddons REQUIRED)
+    find_package(KF${KF_MAJOR_VERSION}WidgetsAddons REQUIRED)
   endif()
 
-  set(KF5_PRIVATE_LIBRARIES
-    KF5::WidgetsAddons
-    KF5::CoreAddons
+  set(KF_PRIVATE_LIBRARIES
+    KF${KF_MAJOR_VERSION}::WidgetsAddons
+    KF${KF_MAJOR_VERSION}::CoreAddons
     )
 endif()

cmake/qet_compilation_vars.cmake

@@ -801,7 +801,7 @@ set(QET_SRC_FILES
   ${QET_DIR}/sources/xml/terminalstriplayoutpatternxml.h
   )
 
-if(NOT BUILD_WITH_KF5)
+if(NOT BUILD_WITH_KF)
   list(APPEND QET_SRC_FILES
     ${QET_DIR}/sources/ui/nokde/kautosavefile.cpp
     ${QET_DIR}/sources/ui/nokde/kautosavefile.h