Assume the following file structure:

CMakeLists.txt
src/A/A.cpp
src/X/X.cpp
src/main.cpp

With the following CMakeLists.txt file:

cmake_minimum_required(VERSION 3.16 FATAL_ERROR)
project("Test")
set(SRC_FILES src/A/A.cpp src/X/X.cpp src/main.cpp)
add_executable(testapp ${SRC_FILES})
source_group(TREE ${CMAKE_CURRENT_SOURCE_DIR} FILES ${SRC_FILES})

This results in main.cpp not being put into "src" group Instead the full path in the file system is displayed. See screenshot.