diff --git a/CLAUDE.md b/CLAUDE.md
index b3c61246..c4871cd9 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -115,9 +115,13 @@ The central data model and control flow (all in `src/classes/` with headers in `
     need no browser.
   - The web assets are installed to `<prefix>/share/musrfit/musrview`; set `MUSRFIT_WEB_DIR` to run
     from the source tree, and `MUSRVIEW_DEBUG=1` to trace the websocket traffic.
-  - musrview needs a ROOT built with `-Droot7=ON -Dwebgui=ON`, plus `-Dqt6web=ON` for the embedded
-    window (`--embedded`, the default). `--browser` uses the system browser, `--browser=server`
-    only prints the URL.
+  - musrview needs a ROOT built with `-Droot7=ON -Dwebgui=ON -Dhttp=ON`, plus `-Dqt6web=ON` for
+    the embedded window (`--embedded`, the default). `--browser` uses the system browser,
+    `--browser=server` only prints the URL. All of that (build options, `ROOT::ROOTWebDisplay`/
+    `ROOT::RHTTP`, the ROOT7 headers, the `ROOT::RWebWindow` API itself via a compile check,
+    JSROOT and its import map) is verified at config time by `cmake/MusrfitCheckROOT7.cmake`,
+    which also sets `MUSRVIEW_HAS_QT6WEB` / `MUSRVIEW_HAS_IMAGE_EXPORT` (the latter gates the
+    `musrview_check` tests, since they compare rendered PNGs and thus need ROOT's libASImage).
 - **`PFourier`** / **`PFourierCanvas`** / **`PPrepFourier`** handle the Fourier transform and its
   (still TCanvas based) display, used by `musrview`/`musrFT`.
 - **`PMsr2Data`** implements `msr2data`'s logic for generating follow-up/global msr files from a
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5371a90d..6a3a017f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -101,16 +101,6 @@ set(MUSRFIT_CXX_FLAGS_NO_ROOT "${CMAKE_CXX_FLAGS}")
 
 find_package(ROOT 6.18 REQUIRED COMPONENTS Gui MathMore Minuit2 XMLParser)
 
-#--- musrview is based on the ROOT7 web graphics, which needs a ROOT built
-#--- with -Dwebgui=ON (and -Droot7=ON). Without it there is no RWebWindow and
-#--- musrview cannot be built. ------------------------------------------------
-if (NOT TARGET ROOT::ROOTWebDisplay)
-  message(FATAL_ERROR
-    "musrview needs the ROOT web display (RWebWindow), which this ROOT "
-    "installation does not provide. Please rebuild ROOT with "
-    "-Droot7=ON -Dwebgui=ON (and -Dqt6web=ON for the embedded window).")
-endif ()
-
 #--- Deliberately NOT 'include(${ROOT_USE_FILE})' here. That file
 #---   - overwrites CMAKE_CXX_STANDARD with ROOT's value, silently discarding
 #---     the setting made at the top of this file,
@@ -143,6 +133,13 @@ add_definitions(${ROOT_DEFINITIONS})
 string(APPEND CMAKE_CXX_FLAGS " ${ROOT_CXX_FLAGS}")
 string(APPEND CMAKE_C_FLAGS " ${ROOT_C_FLAGS}")
 
+#--- musrview is built on the ROOT7 web graphics, which needs a ROOT providing
+#--- quite a bit more than the plain 'find_package(ROOT)' above checks for
+#--- (root7, webgui, http, RWebWindow, JSROOT, ...). All of that is verified
+#--- here, at config time. Done *after* the flags/standard above, so that the
+#--- compile checks in there see exactly the same setup as the real build. ----
+include(MusrfitCheckROOT7)
+
 #--- the next check is need to set a flag, since root 6.24 (minuit2) breaks
 #--- the backwards compatibility. ---------------------------------------------
 if (ROOT_VERSION VERSION_LESS "6.23")
@@ -548,6 +545,20 @@ if (DummyUserFcn)
   message("   PDummyUserFcn                      : yes")
 endif (DummyUserFcn)
 
+message("")
+message(" musrview (ROOT7 web graphics):")
+if (MUSRVIEW_HAS_QT6WEB)
+  message("   embedded window (ROOT qt6web)      : yes")
+else ()
+  message("   embedded window (ROOT qt6web)      : no, will use the standard browser")
+endif ()
+if (MUSRVIEW_HAS_IMAGE_EXPORT)
+  message("   graphics export png/gif/jpg        : yes")
+else ()
+  message("   graphics export png/gif/jpg        : no, ROOT has no libASImage")
+endif ()
+message("   JSROOT                             : ${MUSRVIEW_JSROOT_DIR}")
+
 if (qt_based_tools)
 if (Qt6Core_FOUND)
   message("")
diff --git a/cmake/MusrfitCheckROOT7.cmake b/cmake/MusrfitCheckROOT7.cmake
new file mode 100644
index 00000000..620ec5ae
--- /dev/null
+++ b/cmake/MusrfitCheckROOT7.cmake
@@ -0,0 +1,304 @@
+#--- MusrfitCheckROOT7.cmake --------------------------------------------------
+#
+# Config time checks for everything musrview needs from ROOT.
+#
+# Since musrfit 1.13 musrview is built on the ROOT7 web graphics: the C++ side
+# only holds the data and pushes it as JSON through an ROOT::RWebWindow
+# websocket to a JSROOT page which does the rendering. This requires quite a
+# few ROOT build options to be enabled (root7, webgui, http, ...) which are
+# *not* all on by default in every ROOT installation. Without them musrview
+# either does not compile at all or, worse, compiles and then fails at run time
+# with a blank window. Hence check all of it here, at config time, and tell the
+# user precisely what is missing and how to fix it.
+#
+# Expects find_package(ROOT ...) to have been called already.
+#
+# Sets (cached, for the config summary and for the sources):
+#   MUSRVIEW_JSROOT_DIR      - the JSROOT directory of the ROOT installation
+#   MUSRVIEW_HAS_QT6WEB      - ROOT provides the embedded Qt6 web display
+#   MUSRVIEW_HAS_IMAGE_EXPORT- ROOT provides libASImage (png/gif/jpg export)
+#
+#------------------------------------------------------------------------------
+
+include(CheckCXXSourceCompiles)
+
+if (NOT ROOT_FOUND)
+  message(FATAL_ERROR "MusrfitCheckROOT7: find_package(ROOT) has to be called first.")
+endif ()
+
+#--- collect all problems first, so that the user gets the complete list with
+#--- a single cmake run instead of one error per run --------------------------
+set(_musrview_missing "")
+
+#--- ROOT_BINDIR is set by ROOTConfig.cmake, but be defensive about it since it
+#--- is only used for the messages and for locating the JSROOT directory ------
+if (NOT ROOT_BINDIR)
+  list(GET ROOT_INCLUDE_DIRS 0 _root_incdir)
+  get_filename_component(ROOT_BINDIR "${_root_incdir}/../bin" REALPATH)
+endif ()
+
+#--- cmake does not re-wrap indented lines, hence the texts handed to
+#--- musrview_missing() are wrapped by hand below ----------------------------
+macro(musrview_missing msg)
+  string(APPEND _musrview_missing "   * ${msg}\n")
+endmacro()
+
+#------------------------------------------------------------------------------
+# 1) the ROOT build options musrview relies on
+#
+# ROOTConfig.cmake exports every enabled ROOT build option as
+# ROOT_<option>_FOUND, which is exactly what 'root-config --features' lists.
+#------------------------------------------------------------------------------
+if (NOT ROOT_root7_FOUND)
+  musrview_missing("ROOT was built without '-Droot7=ON' (no ROOT7 classes at all).")
+endif ()
+
+if (NOT ROOT_webgui_FOUND)
+  musrview_missing("ROOT was built without '-Dwebgui=ON' (no RWebWindow/web display).")
+endif ()
+
+if (NOT ROOT_http_FOUND)
+  musrview_missing("ROOT was built without '-Dhttp=ON'. RWebWindow serves its\n\
+     page and its websocket through THttpServer, hence musrview cannot work\n\
+     without it.")
+endif ()
+
+#------------------------------------------------------------------------------
+# 2) the ROOT libraries musrview links against
+#
+# ROOTConfig-targets.cmake only defines the targets which this particular ROOT
+# build actually produced, i.e. a missing target is a reliable statement about
+# the ROOT installation, not about cmake.
+#------------------------------------------------------------------------------
+foreach (_root7_target ROOT::ROOTWebDisplay ROOT::RHTTP)
+  if (NOT TARGET ${_root7_target})
+    musrview_missing("the ROOT library ${_root7_target} is missing.")
+  endif ()
+endforeach ()
+
+#------------------------------------------------------------------------------
+# 3) the ROOT7 headers musrview includes
+#------------------------------------------------------------------------------
+foreach (_root7_header ROOT/RWebWindow.hxx ROOT/RWebWindowsManager.hxx)
+  string(REGEX REPLACE "[/.]" "_" _root7_header_var "MUSRVIEW_HEADER_${_root7_header}")
+  find_file(${_root7_header_var}
+    NAMES ${_root7_header}
+    HINTS ${ROOT_INCLUDE_DIRS}
+    NO_DEFAULT_PATH
+  )
+  mark_as_advanced(${_root7_header_var})
+  if (NOT ${_root7_header_var})
+    musrview_missing("the header <${_root7_header}> is not part of this ROOT\n\
+     installation.")
+  endif ()
+endforeach ()
+
+#------------------------------------------------------------------------------
+# 4) the ROOT7 API musrview uses
+#
+# Checking the headers/libraries is not enough: RWebWindow lived in
+# ROOT::Experimental until ROOT 6.32 and moved to the plain ROOT namespace
+# afterwards, which is what PMusrWebCanvas/musrview use. Compile the actual
+# calls, so that an API change is caught here and not in the middle of the
+# build. CMAKE_TRY_COMPILE_TARGET_TYPE=STATIC_LIBRARY keeps this a compile-only
+# check, i.e. no ROOT libraries have to be dragged in for it.
+#------------------------------------------------------------------------------
+set(MUSRVIEW_ROOT7_API_TEST_SRC "
+#include <memory>
+#include <string>
+#include <TBufferJSON.h>
+#include <TList.h>
+#include <ROOT/RWebWindow.hxx>
+#include <ROOT/RWebWindowsManager.hxx>
+
+int main() {
+  @MUSRVIEW_NS@RWebWindowsManager::SetSingleConnMode(false);
+
+  std::shared_ptr<@MUSRVIEW_NS@RWebWindow> win = @MUSRVIEW_NS@RWebWindow::Create();
+  win->SetDefaultPage(std::string(\"file:musrview.html\"));
+  win->SetGeometry(800, 600);
+  win->SetConnLimit(1);
+  win->SetConnectCallBack([](unsigned) {});
+  win->SetDisconnectCallBack([](unsigned) {});
+  win->SetDataCallBack([](unsigned, const std::string &) {});
+  win->Show(std::string(\"qt6\"));
+  win->Send(1, std::string(\"PLOT:\"));
+  win->CloseConnections();
+
+  TList list;
+  TString json = TBufferJSON::ToJSON(&list, TBufferJSON::kNoSpaces + TBufferJSON::kSameSuppression);
+
+  return json.Length() > 0 ? 0 : 1;
+}
+")
+
+set(CMAKE_REQUIRED_INCLUDES ${ROOT_INCLUDE_DIRS})
+set(CMAKE_REQUIRED_FLAGS "${ROOT_CXX_FLAGS}")
+set(CMAKE_REQUIRED_QUIET TRUE)
+set(_musrview_try_compile_type_save "${CMAKE_TRY_COMPILE_TARGET_TYPE}")
+set(CMAKE_TRY_COMPILE_TARGET_TYPE STATIC_LIBRARY)
+
+set(MUSRVIEW_NS "ROOT::")
+string(CONFIGURE "${MUSRVIEW_ROOT7_API_TEST_SRC}" _musrview_api_src @ONLY)
+check_cxx_source_compiles("${_musrview_api_src}" MUSRVIEW_HAS_ROOT7_API)
+
+if (NOT MUSRVIEW_HAS_ROOT7_API)
+  #--- was it only the namespace, i.e. a ROOT which is too old? --------------
+  set(MUSRVIEW_NS "ROOT::Experimental::")
+  string(CONFIGURE "${MUSRVIEW_ROOT7_API_TEST_SRC}" _musrview_api_src_exp @ONLY)
+  check_cxx_source_compiles("${_musrview_api_src_exp}" MUSRVIEW_HAS_ROOT7_API_EXPERIMENTAL)
+
+  if (MUSRVIEW_HAS_ROOT7_API_EXPERIMENTAL)
+    musrview_missing("this ROOT (${ROOT_VERSION}) still has RWebWindow in the\n\
+     'ROOT::Experimental' namespace. musrview needs the final\n\
+     ROOT::RWebWindow, i.e. ROOT 6.32 or newer.")
+  else ()
+    musrview_missing("the ROOT7 web window API (ROOT::RWebWindow::Create(),\n\
+     SetDefaultPage(), SetGeometry(), SetConnLimit(), SetDataCallBack(),\n\
+     Show(), Send(), ROOT::RWebWindowsManager::SetSingleConnMode(),\n\
+     TBufferJSON::ToJSON()) does not compile against this ROOT\n\
+     (${ROOT_VERSION}). The cmake configure log holds the compiler output.")
+  endif ()
+endif ()
+
+set(CMAKE_TRY_COMPILE_TARGET_TYPE "${_musrview_try_compile_type_save}")
+unset(_musrview_try_compile_type_save)
+unset(CMAKE_REQUIRED_INCLUDES)
+unset(CMAKE_REQUIRED_FLAGS)
+unset(CMAKE_REQUIRED_QUIET)
+unset(MUSRVIEW_NS)
+unset(MUSRVIEW_ROOT7_API_TEST_SRC)
+unset(_musrview_api_src)
+unset(_musrview_api_src_exp)
+
+#------------------------------------------------------------------------------
+# 5) JSROOT
+#
+# The rendering is done by the JSROOT which ships with ROOT. src/musrview_web/
+# musrview.html imports the bare module specifiers 'jsroot/core',
+# 'jsroot/draw' and 'jsroot/webwindow' and relies on THttpServer replacing the
+# '<!--jsroot_importmap-->' marker in the served page by the import map which
+# resolves them. Check both: the JSROOT modules on disk and the import map
+# support of the installed libRHTTP.
+#------------------------------------------------------------------------------
+find_path(MUSRVIEW_JSROOT_DIR
+  NAMES modules/core.mjs
+  HINTS $ENV{JSROOTSYS}
+        ${ROOT_BINDIR}/../js
+        ${ROOT_BINDIR}/../share/root/js
+        $ENV{ROOTSYS}/js
+  NO_DEFAULT_PATH
+  DOC "JSROOT directory of the ROOT installation used by musrview"
+)
+mark_as_advanced(MUSRVIEW_JSROOT_DIR)
+
+if (NOT MUSRVIEW_JSROOT_DIR)
+  musrview_missing("the JSROOT of this ROOT installation could not be found\n\
+     (no 'js/modules/core.mjs'). musrview is rendered by JSROOT, hence it is\n\
+     needed. If it sits somewhere else, point JSROOTSYS or\n\
+     -DMUSRVIEW_JSROOT_DIR=<dir> at it.")
+else ()
+  foreach (_jsroot_module core.mjs draw.mjs webwindow.mjs)
+    if (NOT EXISTS "${MUSRVIEW_JSROOT_DIR}/modules/${_jsroot_module}")
+      musrview_missing("the JSROOT module '${_jsroot_module}' is missing in\n\
+     ${MUSRVIEW_JSROOT_DIR}/modules, but musrview.html imports it.")
+    endif ()
+  endforeach ()
+endif ()
+
+#--- The import map itself is generated by THttpServer, i.e. by libRHTTP.
+#--- Two independent signals that this ROOT supports it: the pages ROOT itself
+#--- serves carry the very same marker, and libRHTTP holds the marker together
+#--- with the 'jsroot/...' module specifiers it maps (file(STRINGS ...) also
+#--- works on binaries). Either one is good enough, complain only if neither is
+#--- there - which is much more robust than deducing it from the ROOT version.
+set(_musrview_has_importmap FALSE)
+
+#--- signal 1: a page shipped by ROOT which is served through THttpServer ----
+if (MUSRVIEW_JSROOT_DIR AND EXISTS "${MUSRVIEW_JSROOT_DIR}/files/online.htm")
+  file(STRINGS "${MUSRVIEW_JSROOT_DIR}/files/online.htm" _jsroot_marker
+       REGEX "jsroot_importmap")
+  if (_jsroot_marker)
+    set(_musrview_has_importmap TRUE)
+  endif ()
+endif ()
+
+#--- signal 2: libRHTTP itself ----------------------------------------------
+if (NOT _musrview_has_importmap AND TARGET ROOT::RHTTP)
+  set(_rhttp_lib "")
+  foreach (_prop IMPORTED_LOCATION IMPORTED_LOCATION_RELEASE IMPORTED_LOCATION_NOCONFIG)
+    if (NOT _rhttp_lib)
+      get_target_property(_rhttp_lib ROOT::RHTTP ${_prop})
+      if (NOT _rhttp_lib)
+        set(_rhttp_lib "")
+      endif ()
+    endif ()
+  endforeach ()
+
+  if (_rhttp_lib AND EXISTS "${_rhttp_lib}")
+    file(STRINGS "${_rhttp_lib}" _rhttp_importmap REGEX "jsroot_importmap")
+    file(STRINGS "${_rhttp_lib}" _rhttp_webwindow REGEX "^jsroot/webwindow$")
+    if (_rhttp_importmap AND _rhttp_webwindow)
+      set(_musrview_has_importmap TRUE)
+    endif ()
+  else ()
+    #--- nothing to look at, do not cry wolf ---------------------------------
+    set(_musrview_has_importmap TRUE)
+  endif ()
+endif ()
+
+if (NOT _musrview_has_importmap)
+  musrview_missing("the THttpServer of this ROOT does not provide the JSROOT\n\
+     import map ('<!--jsroot_importmap-->' and the 'jsroot/...' module\n\
+     specifiers) which src/musrview_web/musrview.html relies on. ROOT 6.32 or\n\
+     newer is required.")
+endif ()
+
+#------------------------------------------------------------------------------
+# 6) bail out if anything is missing
+#------------------------------------------------------------------------------
+if (_musrview_missing)
+  get_filename_component(_root_prefix "${ROOT_BINDIR}/.." REALPATH)
+  message(FATAL_ERROR
+    "musrview is built on the ROOT7 web graphics, but the ROOT installation\n"
+    "   ${_root_prefix}  (version ${ROOT_VERSION})\n"
+    "does not provide everything it needs:\n"
+    "${_musrview_missing}"
+    "Please rebuild ROOT with at least\n"
+    "   -Droot7=ON -Dwebgui=ON -Dhttp=ON\n"
+    "and, for the embedded musrview window, in addition -Dqt6web=ON.\n"
+    "If more than one ROOT is installed, check that ROOTSYS/CMAKE_PREFIX_PATH point to the right one.")
+endif ()
+
+#------------------------------------------------------------------------------
+# 7) optional, i.e. musrview works without it but with reduced functionality
+#------------------------------------------------------------------------------
+
+#--- embedded window: '--embedded' is musrview's default, without the Qt6 web
+#--- display of ROOT it falls back to the standard browser of the system. -----
+if (TARGET ROOT::ROOTQt6WebDisplay AND ROOT_qt6web_FOUND)
+  set(MUSRVIEW_HAS_QT6WEB TRUE CACHE INTERNAL "ROOT provides the embedded Qt6 web display")
+else ()
+  set(MUSRVIEW_HAS_QT6WEB FALSE CACHE INTERNAL "ROOT provides the embedded Qt6 web display")
+  message(WARNING
+    "ROOT is built without the Qt6 web display (-Dqt6web=ON), hence musrview cannot "
+    "show its embedded window and will use the standard browser of the system instead. "
+    "Everything else works as usual.")
+endif ()
+
+#--- graphics export: musrview --png/--gif/--jpg draw into a real TCanvas in
+#--- batch mode, which needs libASImage. eps/pdf/svg/root do not. The
+#--- musrview_check tests compare png output, i.e. they need it as well. ------
+if (TARGET ROOT::ASImage AND ROOT_asimage_FOUND)
+  set(MUSRVIEW_HAS_IMAGE_EXPORT TRUE CACHE INTERNAL "ROOT provides libASImage")
+else ()
+  set(MUSRVIEW_HAS_IMAGE_EXPORT FALSE CACHE INTERNAL "ROOT provides libASImage")
+  message(WARNING
+    "ROOT is built without libASImage (-Dasimage=ON), hence 'musrview --png/--gif/--jpg' "
+    "will not produce any output (eps/pdf/svg/root are fine) and the musrview_check "
+    "tests will fail.")
+endif ()
+
+unset(_musrview_missing)
+unset(_musrview_has_importmap)
+unset(_rhttp_lib)
diff --git a/tests/musrview_check/CMakeLists.txt b/tests/musrview_check/CMakeLists.txt
index e2a2e4a2..109bac5a 100644
--- a/tests/musrview_check/CMakeLists.txt
+++ b/tests/musrview_check/CMakeLists.txt
@@ -1,5 +1,14 @@
 #--- musrview PNG integration tests -------------------------------------------
 
+#--- these tests compare the PNG which 'musrview --png' renders. Without
+#--- libASImage ROOT cannot write a PNG at all (see cmake/MusrfitCheckROOT7.cmake,
+#--- which warns about it at config time), hence do not register tests which
+#--- could then only fail for a reason that has nothing to do with musrfit. ---
+if (NOT MUSRVIEW_HAS_IMAGE_EXPORT)
+  message(STATUS "musrview_check: ROOT has no libASImage, skipping the musrview-* tests")
+  return ()
+endif ()
+
 find_package(Python3 REQUIRED COMPONENTS Interpreter)
 
 #--- musrview_check.py needs Pillow/numpy for the pixel comparison. CMake's