Load view_to_iframe iframes eagerly by default

`view_to_iframe` previously forced `loading="lazy"` on the rendered iframe.
The browser only fetches a lazy iframe's document once it is scrolled close to
the viewport. On the home page these iframes sit well below the fold, and the
test harness performs no navigation (and no scroll) when it is already on "/",
so a deferred lazy iframe is never fetched and a consumer waiting on the
iframe's contents times out (observed intermittently on slow CI runners, e.g.
`test_component_view_to_iframe_sync_func` on windows-latest).

An eager iframe (the HTML default, no `loading` attribute) is a sub-resource
the browser fetches as part of the parent page's initial load, so its document
is guaranteed to be present as soon as the page loads. This makes loading
deterministic instead of depending on the browser's load-timing heuristics,
which is the real root cause of the flake.

Lazy loading remains available as an opt-in for users who prefer it: pass
`extra_props={"loading": "lazy"}`.

Author: User

CHANGELOG.md

@@ -31,6 +31,7 @@ Don't forget to remove deprecated code on each major release!
 
 ### Changed
 
+- `view_to_iframe` now renders its `iframe` without a `loading` attribute, so it loads **eagerly** (the HTML default) instead of lazily. The `iframe`'s document is now fetched as part of the parent page's initial load and is guaranteed to be available as soon as the page loads, rather than being deferred until it is scrolled close to the viewport. To restore the previous lazy-loading behavior, pass `extra_props={"loading": "lazy"}`.
 - Use one WebSocket per client webpage.
 - Updated dependencies: `reactpy>=2.0.0, <3.0.0` and `reactpy-router>=3.0.0, <4.0.0`.
 - Updated Python support to 3.11–3.14.

docs/src/reference/components.md

@@ -301,6 +301,10 @@ Compatible with sync or async [Function Based Views](https://docs.djangoproject.
     - The `#!python iframe` will always load **after** the parent component.
     - CSS styling for `#!python iframe` elements tends to be awkward.
 
+??? info "Loading behavior"
+
+    The `#!python iframe` loads **eagerly** by default: its document is fetched as part of the parent page's initial load, so it is available as soon as the page loads. If you want the browser to defer fetching the `#!python iframe`'s document until it is scrolled close to the viewport, pass `#!python extra_props={"loading": "lazy"}`.
+
 ??? question "How do I use this for Class Based Views?"
 
     Class Based Views are accepted by `#!python view_to_iframe` as an argument.

src/reactpy_django/components.py

@@ -69,6 +69,12 @@ def constructor(
 
 def view_to_iframe(view: Callable | View | str, extra_props: dict[str, Any] | None = None) -> ViewToIframeConstructor:
     """
+    Converts a Django view into a ReactPy component that is rendered within an `iframe` element.
+
+    The `iframe` loads eagerly by default, so its document is fetched as part of the parent page's
+    initial load and is available as soon as the page loads. If you want the browser to defer the
+    iframe's fetch until it is scrolled close to the viewport, pass `extra_props={"loading": "lazy"}`.
+
     Args:
         view: The view function or class to convert, or the dotted path to the view.
 
@@ -244,11 +250,17 @@ def _view_to_iframe(
     extra_props = extra_props or {}
     extra_props.pop("src", None)
 
+    # The iframe loads eagerly by default (the HTML default). An eager iframe is
+    # a sub-resource that the browser fetches as part of the parent page's
+    # initial load, so its document is reliably present as soon as the page
+    # loads. The previous default of `loading="lazy"` deferred the fetch until
+    # the iframe was scrolled close to the viewport, which made availability
+    # non-deterministic. If you want that deferred behavior, opt in via
+    # `extra_props={"loading": "lazy"}`.
     return html.iframe(
         {
             "src": reverse("reactpy:view_to_iframe", args=[dotted_path]) + query_string,
             "style": {"border": "none"},
-            "loading": "lazy",
         }
         | extra_props
     )