source/blender/blenkernel/BKE_blender_project.h

@@ -27,12 +27,17 @@ BlenderProject *BKE_project_active_get(void) ATTR_WARN_UNUSED_RESULT;
  */
 void BKE_project_active_unset(void);
 /**
- * Attempt to load and activate a project based on the given path. If the path doesn't lead into a
- * project, the active project is unset. Note that the project will be unset on any failure when
- * loading the project.
+ * Check if \a path points into the project root path (i.e. if one of the ancestors of the
+ * referenced file/directory is a project root directory).
+ */
+bool BKE_project_contains_path(const char *path) ATTR_WARN_UNUSED_RESULT ATTR_NONNULL();
+/**
+ * Attempt to load and activate a project based on the given path. If the path doesn't lead
+ * into a project, the active project is unset. Note that the project will be unset on any
+ * failure when loading the project.
  *
- * \note: When setting an active project, the previously active one will be destroyed, so pointers
- *        may dangle.
+ * \note: When setting an active project, the previously active one will be destroyed, so
+ * pointers may dangle.
  */
 BlenderProject *BKE_project_active_load_from_path(const char *path) ATTR_NONNULL();
 

source/blender/blenkernel/intern/blender_project.cc

@@ -296,6 +296,12 @@ void BKE_project_active_unset(void)
   bke::BlenderProject::set_active_from_settings(nullptr);
 }
 
+bool BKE_project_contains_path(const char *path)
+{
+  const StringRef found_root_path = bke::BlenderProject::project_root_path_find_from_path(path);
+  return !found_root_path.is_empty();
+}
+
 BlenderProject *BKE_project_active_load_from_path(const char *path)
 {
   /* Project should be unset if the path doesn't contain a project root. Unset in the beginning so

source/blender/windowmanager/intern/wm_files.c

@@ -3152,6 +3152,16 @@ static int wm_save_as_mainfile_exec(bContext *C, wmOperator *op)
     return OPERATOR_CANCELLED;
   }
 
+  const BlenderProject *active_project = CTX_wm_project();
+  if (active_project && !BKE_project_contains_path(path)) {
+    BKE_reportf(
+        op->reports,
+        RPT_WARNING,
+        "File saved outside of the active project's path (\"%s\"). Active project changed.",
+        BKE_project_root_path_get(active_project));
+    /* Don't cancel. Otherwise there's no way to save files outside of the active project. */
+  }
+
   const int fileflags_orig = G.fileflags;
   int fileflags = G.fileflags;