diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000000..81a2de4370
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,3 @@
+# The base64 tests assert on this fixture's exact bytes, so it must not be
+# rewritten to CRLF on checkout (the default on Windows with core.autocrlf).
+tests/sample_file.txt -text
diff --git a/tests/test_client.py b/tests/test_client.py
index 79a66098fa..5c29f246de 100644
--- a/tests/test_client.py
+++ b/tests/test_client.py
@@ -1324,8 +1324,9 @@ def retry_handler(_request: httpx2.Request) -> httpx2.Response:
 
     def test_proxy_environment_variables(self, monkeypatch: pytest.MonkeyPatch) -> None:
         # Test that the proxy environment variables are set correctly
-        monkeypatch.setenv("HTTPS_PROXY", "https://example.org")
-        # Delete in case our environment has any proxy env vars set
+        # Delete in case our environment has any proxy env vars set. This must
+        # happen before setting HTTPS_PROXY: os.environ is case-insensitive on
+        # Windows, so deleting "https_proxy" afterwards would remove it again.
         monkeypatch.delenv("HTTP_PROXY", raising=False)
         monkeypatch.delenv("ALL_PROXY", raising=False)
         monkeypatch.delenv("NO_PROXY", raising=False)
@@ -1333,6 +1334,7 @@ def test_proxy_environment_variables(self, monkeypatch: pytest.MonkeyPatch) -> N
         monkeypatch.delenv("https_proxy", raising=False)
         monkeypatch.delenv("all_proxy", raising=False)
         monkeypatch.delenv("no_proxy", raising=False)
+        monkeypatch.setenv("HTTPS_PROXY", "https://example.org")
 
         client = DefaultHttpxClient()
 
@@ -2625,8 +2627,9 @@ async def test_get_platform(self) -> None:
 
     async def test_proxy_environment_variables(self, monkeypatch: pytest.MonkeyPatch) -> None:
         # Test that the proxy environment variables are set correctly
-        monkeypatch.setenv("HTTPS_PROXY", "https://example.org")
-        # Delete in case our environment has any proxy env vars set
+        # Delete in case our environment has any proxy env vars set. This must
+        # happen before setting HTTPS_PROXY: os.environ is case-insensitive on
+        # Windows, so deleting "https_proxy" afterwards would remove it again.
         monkeypatch.delenv("HTTP_PROXY", raising=False)
         monkeypatch.delenv("ALL_PROXY", raising=False)
         monkeypatch.delenv("NO_PROXY", raising=False)
@@ -2634,6 +2637,7 @@ async def test_proxy_environment_variables(self, monkeypatch: pytest.MonkeyPatch
         monkeypatch.delenv("https_proxy", raising=False)
         monkeypatch.delenv("all_proxy", raising=False)
         monkeypatch.delenv("no_proxy", raising=False)
+        monkeypatch.setenv("HTTPS_PROXY", "https://example.org")
 
         client = DefaultAsyncHttpxClient()