Add test for self-hosted docs behind a proxy

tests/test_offline_docs/static/swagger.css

@@ -0,0 +1 @@
+.swagger-ui{color:#3b4151;}

tests/test_offline_docs/static/swagger.js

@@ -0,0 +1,3 @@
+function foo() {
+    return "foo"
+}

tests/test_offline_docs/test_root_path_with_static_mount.py

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+from fastapi import FastAPI
+from fastapi.openapi.docs import get_swagger_ui_html
+from fastapi.staticfiles import StaticFiles
+from fastapi.testclient import TestClient
+
+root_path = "/api"
+
+app = FastAPI(
+    title="FastAPI",
+    root_path=root_path,
+    docs_url=None,
+    redoc_url=None,
+)
+
+app.mount(
+    "/static", StaticFiles(directory="tests/test_offline_docs/static"), name="static"
+)
+
+
+@app.get("/")
+async def custom_swagger_ui_html():
+    """
+    Sets up a localized version of the Swagger (OpenAPI) docs that can be run without assets from the Internet.
+    """
+
+    return get_swagger_ui_html(
+        openapi_url="/openapi.json",
+        title=app.title,
+        swagger_js_url="/static/swagger.js",
+        swagger_css_url="/static/swagger.css",
+    )
+
+
+client = TestClient(app)
+
+
+def test_static_assets():
+    """
+    Verify static assets can still be loaded properly even behind a proxy (root_path)
+    """
+    response = client.get("/")
+    assert response.status_code == 200
+    assert response.headers["Content-Type"] == "text/html; charset=utf-8"
+    swagger_html = response.text
+
+    response = client.get("/openapi.json")
+    assert response.status_code == 200
+
+    response = client.get("/static/swagger.js")
+    assert response.status_code == 200
+
+    response = client.get("/static/swagger.css")
+    assert response.status_code == 200
+
+    assert "/static/swagger.js" in swagger_html
+    assert f"{root_path}/static/swagger.js" not in swagger_html
+    assert "/static/swagger.css" in swagger_html
+    assert f"{root_path}/static/swagger.css" not in swagger_html