diff --git a/deps/CMakeLists.txt b/deps/CMakeLists.txt
index 967f47aac2..8672a50310 100644
--- a/deps/CMakeLists.txt
+++ b/deps/CMakeLists.txt
@@ -155,7 +155,7 @@ if (NOT _is_multi AND NOT CMAKE_BUILD_TYPE)
 endif ()
 
 function(orcaslicer_add_cmake_project projectname)
-    cmake_parse_arguments(P_ARGS "FORWARD_CONFIG" "INSTALL_DIR;BUILD_COMMAND;INSTALL_COMMAND" "CMAKE_ARGS" ${ARGN})
+    cmake_parse_arguments(P_ARGS "FORWARD_CONFIG" "INSTALL_DIR;BUILD_COMMAND;INSTALL_COMMAND;SOURCE_DIR" "CMAKE_ARGS" ${ARGN})
 
     # MSVC is true for clang-cl as well, so the sub-build toolchain has to key on the
     # generator. A non-Visual-Studio superbuild passes its own generator down, and with
@@ -196,12 +196,18 @@ function(orcaslicer_add_cmake_project projectname)
       set(_build_j "-j${NPROC}")
     endif ()
 
+    set(_source_dir_arg "")
+    if (P_ARGS_SOURCE_DIR)
+        set(_source_dir_arg SOURCE_DIR ${P_ARGS_SOURCE_DIR})
+    endif ()
+
 if (NOT IS_CROSS_COMPILE OR NOT APPLE)
     ExternalProject_Add(
         dep_${projectname}
         EXCLUDE_FROM_ALL    ON
         INSTALL_DIR         ${DESTDIR}
         DOWNLOAD_DIR        ${DEP_DOWNLOAD_DIR}/${projectname}
+        ${_source_dir_arg}
         ${_gen}
         CMAKE_ARGS
             -DCMAKE_POLICY_VERSION_MINIMUM=3.5
@@ -234,12 +240,14 @@ if (NOT IS_CROSS_COMPILE OR NOT APPLE)
         # note for future devs: shared libs may actually create a size reduction
         # but orcaslicer_deps tends to get really funny regarding linking after that (notably boost)
         # so, as much as I would like to use that, it's not happening
-        ExternalProject_Add_Step(dep_${projectname} free_download_space
-                DEPENDEES download # do after download
-                COMMENT "Freeing Space: Removing source archive"
-                WORKING_DIRECTORY ${DEP_DOWNLOAD_DIR}
-                COMMAND ${CMAKE_COMMAND} -E rm -rf ${projectname}
-        )
+        if (NOT P_ARGS_SOURCE_DIR)
+            ExternalProject_Add_Step(dep_${projectname} free_download_space
+                    DEPENDEES download # do after download
+                    COMMENT "Freeing Space: Removing source archive"
+                    WORKING_DIRECTORY ${DEP_DOWNLOAD_DIR}
+                    COMMAND ${CMAKE_COMMAND} -E rm -rf ${projectname}
+            )
+        endif ()
         ExternalProject_Add_Step(dep_${projectname} free_build_space
                 DEPENDEES install # do after install
                 COMMENT "Freeing Space: Removing source and build files"
@@ -253,6 +261,7 @@ else()
         EXCLUDE_FROM_ALL    ON
         INSTALL_DIR         ${DESTDIR}
         DOWNLOAD_DIR        ${DEP_DOWNLOAD_DIR}/${projectname}
+        ${_source_dir_arg}
         ${_gen}
         CMAKE_ARGS
             -DCMAKE_POLICY_VERSION_MINIMUM=3.5
diff --git a/deps/DataChannel/DataChannel.cmake b/deps/DataChannel/DataChannel.cmake
index 23c256b365..c5ab8a8f1a 100644
--- a/deps/DataChannel/DataChannel.cmake
+++ b/deps/DataChannel/DataChannel.cmake
@@ -5,12 +5,13 @@
 # It vendors plog, usrsctp and libjuice as git submodules, which a plain
 # GitHub tag tarball does not include. The flatpak sandbox has no network
 # access during the build, so there the manifest itself clones the repo
-# (submodules and all) straight into this ExternalProject's default source
-# dir before the sandbox closes; with no download method given here and that
-# dir already non-empty, ExternalProject_Add silently uses it as-is instead
-# of trying its own (network) git step.
+# (submodules and all) into the dependency download directory before the
+# sandbox closes. ExternalProject_Add is pointed at that existing checkout
+# instead of being given its own network-dependent download method.
 if (FLATPAK)
-    set(_datachannel_source "")
+    set(_datachannel_source
+        SOURCE_DIR ${DEP_DOWNLOAD_DIR}/DataChannel
+    )
 else()
     set(_datachannel_source
         GIT_REPOSITORY https://github.com/paullouisageneau/libdatachannel.git
diff --git a/scripts/flatpak/com.orcaslicer.OrcaSlicer.yml b/scripts/flatpak/com.orcaslicer.OrcaSlicer.yml
index e6aa25d88c..7cd432ee68 100644
--- a/scripts/flatpak/com.orcaslicer.OrcaSlicer.yml
+++ b/scripts/flatpak/com.orcaslicer.OrcaSlicer.yml
@@ -326,16 +326,13 @@ modules:
         dest: external-packages/FFMPEG
 
       # libdatachannel v0.24.5
-      # Use git instead of archive: it vendors plog, usrsctp and libjuice as
-      # git submodules, which a plain GitHub tag tarball does not include.
-      # Cloned (with submodules) straight into the ExternalProject source dir
-      # that deps/DataChannel/DataChannel.cmake expects, since the sandboxed
-      # build step has no network to do this itself.
+      # The Git checkout includes the submodules that are missing from the
+      # GitHub source archive. DataChannel.cmake consumes it as SOURCE_DIR.
       - type: git
         url: https://github.com/paullouisageneau/libdatachannel.git
         tag: v0.24.5
         commit: 443f6934d9007eb7076ab7825ba330f355fcbead
-        dest: deps/build_flatpak/dep_DataChannel-prefix/src/dep_DataChannel
+        dest: external-packages/DataChannel
 
       # ---------------------------------------------------------------
       # Fallback archives for deps normally provided by the GNOME SDK.