Fix plain section shadows env config

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>

docs/changelog/2636.bugfix.rst

@@ -0,0 +1,2 @@
+A plain section in INI configuration matching a tox environment name shadowed the laters configuration - by
+:user:`gaborbernat`.

src/tox/config/source/ini.py

@@ -85,8 +85,8 @@ def register_factors(envs: Iterable[str]) -> None:
         for section in self.sections():
             register_factors(section.names)
             for name in section.names:
-                self._section_mapping[name].append(section.key)
                 if section.is_test_env:
+                    self._section_mapping[name].append(section.key)
                     yield name
         # add all conditional markers that are not part of the explicitly defined sections
         for section in self.sections():

tests/session/cmd/test_show_config.py

@@ -233,3 +233,16 @@ def test_show_config_core_host_python(tox_project: ToxProjectCreator) -> None:
     outcome = project.run("c", "--core", "-e", "py", "-k", "host_python")
     outcome.assert_success()
     assert f"host_python = {sys.executable}" in outcome.out
+
+
+def test_show_config_matching_env_section(tox_project: ToxProjectCreator) -> None:
+    ini = """
+    [a]
+    [testenv:a]
+    deps = c>=1
+    [testenv:b]
+    deps = {[testenv:a]deps}"""
+    project = tox_project({"tox.ini": ini})
+    outcome = project.run("c", "-e", "a,b", "-k", "deps")
+    outcome.assert_success()
+    assert outcome.out.count("c>=1") == 2, outcome.out