diff --git a/.github/workflows/core.yml b/.github/workflows/core.yml
index 6f21021a..0a0e9a16 100644
--- a/.github/workflows/core.yml
+++ b/.github/workflows/core.yml
@@ -8,14 +8,19 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        exclude:
+          - os: macos-latest
+            python-version: "3.6"
+          - os: windows-latest
+            python-version: "3.10"
     runs-on: ${{ matrix.os }}
     steps:
       - uses: actions/checkout@v2
         with:
           submodules: "recursive"
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v1
+        uses: actions/setup-python@v2
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install dependencies
diff --git a/HISTORY.rst b/HISTORY.rst
index d84c84da..1089d25d 100644
--- a/HISTORY.rst
+++ b/HISTORY.rst
@@ -8,6 +8,14 @@ PlatformIO Core 5
 
 **A professional collaborative platform for embedded development**
 
+5.2.2 (2021-10-20)
+~~~~~~~~~~~~~~~~~~
+
+- Override debugging firmware loading mode using ``--load-mode`` option for `pio debug <https://docs.platformio.org/en/latest/core/userguide/cmd_debug.html>`__ command
+- Added support for CLion IDE 2021.3 (`pull #4085 <https://github.com/platformio/platformio-core/issues/4085>`_)
+- Removed debugging "legacy Click" message from CLI (`issue #4083 <https://github.com/platformio/platformio-core/issues/4083>`_)
+- Fixed a "TypeError: sequence item 1: expected str instance, list found" issue when extending configuration option in `"platformio.ini" <https://docs.platformio.org/page/projectconf.html>`__ with the multi-line default value (`issue #4082 <https://github.com/platformio/platformio-core/issues/4082>`_)
+
 5.2.1 (2021-10-11)
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/docs b/docs
index 8a613430..66f67cb3 160000
--- a/docs
+++ b/docs
@@ -1 +1 @@
-Subproject commit 8a613430957704dfc2e4fc30337020a0a80cfba8
+Subproject commit 66f67cb3357265ab52a0567b67337ca7fda6c3e4
diff --git a/platformio/__init__.py b/platformio/__init__.py
index 31da28ca..56d45672 100644
--- a/platformio/__init__.py
+++ b/platformio/__init__.py
@@ -14,7 +14,7 @@
 
 import sys
 
-VERSION = (5, 2, 1)
+VERSION = (5, 2, 2)
 __version__ = ".".join([str(s) for s in VERSION])
 
 __title__ = "platformio"
diff --git a/platformio/__main__.py b/platformio/__main__.py
index f2c38808..f7bdd61b 100644
--- a/platformio/__main__.py
+++ b/platformio/__main__.py
@@ -76,7 +76,6 @@ try:
 
 
 except (AttributeError, TypeError):  # legacy support for CLick > 8.0.1
-    print("legacy Click")
 
     @cli.resultcallback()
     @click.pass_context
diff --git a/platformio/commands/debug.py b/platformio/commands/debug.py
index d0cbf233..48d86891 100644
--- a/platformio/commands/debug.py
+++ b/platformio/commands/debug.py
@@ -17,6 +17,7 @@
 
 import asyncio
 import os
+import signal
 import subprocess
 
 import click
@@ -31,6 +32,7 @@ from platformio.debug.process.gdb import GDBClientProcess
 from platformio.project.config import ProjectConfig
 from platformio.project.exception import ProjectEnvsNotAvailableError
 from platformio.project.helpers import is_platformio_project
+from platformio.project.options import ProjectOptions
 
 
 @click.command(
@@ -54,11 +56,21 @@ from platformio.project.helpers import is_platformio_project
     ),
 )
 @click.option("--environment", "-e", metavar="<environment>")
+@click.option("--load-mode", type=ProjectOptions["env.debug_load_mode"].type)
 @click.option("--verbose", "-v", is_flag=True)
 @click.option("--interface", type=click.Choice(["gdb"]))
 @click.argument("__unprocessed", nargs=-1, type=click.UNPROCESSED)
 @click.pass_context
-def cli(ctx, project_dir, project_conf, environment, verbose, interface, __unprocessed):
+def cli(
+    ctx,
+    project_dir,
+    project_conf,
+    environment,
+    load_mode,
+    verbose,
+    interface,
+    __unprocessed,
+):
     app.set_session_var("custom_project_conf", project_conf)
 
     # use env variables from Eclipse or CLion
@@ -104,7 +116,7 @@ def cli(ctx, project_dir, project_conf, environment, verbose, interface, __unpro
 
     rebuild_prog = False
     preload = debug_config.load_cmds == ["preload"]
-    load_mode = debug_config.load_mode
+    load_mode = load_mode or debug_config.load_mode
     if load_mode == "always":
         rebuild_prog = preload or not helpers.has_debug_symbols(
             debug_config.program_path
@@ -155,6 +167,7 @@ def cli(ctx, project_dir, project_conf, environment, verbose, interface, __unpro
         client = GDBClientProcess(project_dir, debug_config)
         coro = client.run(__unprocessed)
         try:
+            signal.signal(signal.SIGINT, signal.SIG_IGN)
             loop.run_until_complete(coro)
             if IS_WINDOWS:
                 # an issue with `asyncio` executor and STIDIN,
diff --git a/platformio/ide/tpls/clion/CMakeListsPrivate.txt.tpl b/platformio/ide/tpls/clion/CMakeListsPrivate.txt.tpl
index a5c0010b..9df294ee 100644
--- a/platformio/ide/tpls/clion/CMakeListsPrivate.txt.tpl
+++ b/platformio/ide/tpls/clion/CMakeListsPrivate.txt.tpl
@@ -75,7 +75,7 @@ set(CMAKE_CXX_STANDARD {{ cxx_stds[-1] }})
 
 if (CMAKE_BUILD_TYPE MATCHES "{{ env_name }}")
 % for define in defines:
-    add_definitions(-D'{{!re.sub(r"([\"\(\)#])", r"\\\1", define)}}')
+    add_definitions(-D{{!re.sub(r"([\"\(\)#])", r"\\\1", define)}})
 % end
 
 % for include in filter_includes(includes):
@@ -99,7 +99,7 @@ endif()
 % for env, data in ide_data.items():
 if (CMAKE_BUILD_TYPE MATCHES "{{ env }}")
 %   for define in data["defines"]:
-    add_definitions(-D'{{!re.sub(r"([\"\(\)#])", r"\\\1", define)}}')
+    add_definitions(-D{{!re.sub(r"([\"\(\)#])", r"\\\1", define)}})
 %   end
 
 %   for include in filter_includes(data["includes"]):
diff --git a/platformio/ide/tpls/vscode/.vscode/launch.json.tpl b/platformio/ide/tpls/vscode/.vscode/launch.json.tpl
index e05c7237..eb654808 100644
--- a/platformio/ide/tpls/vscode/.vscode/launch.json.tpl
+++ b/platformio/ide/tpls/vscode/.vscode/launch.json.tpl
@@ -30,7 +30,10 @@
 %    "type": "PlatformIO",
 %    "task": ("Pre-Debug (%s)" % env_name) if len(config.envs()) > 1 else "Pre-Debug",
 %  }
-%  return [debug, predebug]
+%  noloading = predebug.copy()
+%  noloading["name"] = "PIO Debug (without uploading)"
+%  noloading["loadMode"] = "manual"
+%  return [debug, predebug, noloading]
 % end
 %
 % def _remove_comments(lines):
diff --git a/platformio/project/config.py b/platformio/project/config.py
index 13ff8e2b..187cb993 100644
--- a/platformio/project/config.py
+++ b/platformio/project/config.py
@@ -295,7 +295,10 @@ class ProjectConfigBase(object):
         section, option = match.group(1), match.group(2)
         if section == "sysenv":
             return os.getenv(option)
-        return self.getraw(section, option)
+        value = self.getraw(section, option)
+        if isinstance(value, list):
+            return "\n".join(value)
+        return value
 
     def get(self, section, option, default=MISSING):
         value = None
diff --git a/tests/test_projectconf.py b/tests/test_projectconf.py
index 832ceb71..99c71ec0 100644
--- a/tests/test_projectconf.py
+++ b/tests/test_projectconf.py
@@ -63,6 +63,10 @@ targets =
 [env:test_extends]
 extends = strict_settings
 
+[env:inject_base_env]
+debug_build_flags =
+    ${env.debug_build_flags}
+    -D CUSTOM_DEBUG_FLAG
 
 """
 
@@ -150,13 +154,20 @@ def test_sections(config):
         "custom",
         "env:base",
         "env:test_extends",
+        "env:inject_base_env",
         "env:extra_1",
         "env:extra_2",
     ]
 
 
 def test_envs(config):
-    assert config.envs() == ["base", "test_extends", "extra_1", "extra_2"]
+    assert config.envs() == [
+        "base",
+        "test_extends",
+        "inject_base_env",
+        "extra_1",
+        "extra_2",
+    ]
     assert config.default_envs() == ["base", "extra_2"]
 
 
@@ -274,6 +285,14 @@ def test_get_value(config):
     assert config.get("env:extra_2", "monitor_speed") == 9600
     assert config.get("env:base", "build_flags") == ["-D DEBUG=1"]
 
+    # get default value from ConfigOption
+    assert config.get("env:inject_base_env", "debug_build_flags") == [
+        "-Og",
+        "-g2",
+        "-ggdb2",
+        "-D CUSTOM_DEBUG_FLAG",
+    ]
+
 
 def test_items(config):
     assert config.items("custom") == [
@@ -445,4 +464,13 @@ def test_dump(tmpdir_factory):
             ],
         ),
         ("env:test_extends", [("extends", ["strict_settings"])]),
+        (
+            "env:inject_base_env",
+            [
+                (
+                    "debug_build_flags",
+                    ["${env.debug_build_flags}", "-D CUSTOM_DEBUG_FLAG"],
+                )
+            ],
+        ),
     ]