The panel types `StreamFieldPanel`, `RichTextFieldPanel`, `ImageChooserPanel`, `DocumentChooserPanel` and `SnippetChooserPanel` have been phased out, and can now be replaced with `FieldPanel`. Additionally, `PageChooserPanel` is only required when passing a `page_type` or `can_choose_root`, and can otherwise be replaced with `FieldPanel`. In all cases, `FieldPanel` will now automatically select the most appropriate form element. This feature was developed by Matt Westcott.
[`FieldPanel`](wagtail.admin.edit_handlers.FieldPanel) now accepts a `permission` keyword argument to specify that the field should only be available to users with a given permission level. This feature was developed by Matt Westcott and sponsored by Google as part of Wagtail's page editor redevelopment.
* Major updates to frontend tooling; move Node tooling from Gulp to Webpack, upgrade to Node v16 and npm v8, eslint v8, stylelint v14 and others (Thibaud Colas)
* When Documents (e.g. PDFs) have been configured to be served inline via `WAGTAILDOCS_CONTENT_TYPES`&`WAGTAILDOCS_INLINE_CONTENT_TYPES` ensure that the filename is correctly set in the `Content-Disposition` header so that saving the files will use the correct filename (John-Scott Atlakson)
* IE11 support was officially dropped in Wagtail 2.15, as of this release there will no longer be a warning shown to users of this browser.
* Wagtail is fully compatible with Microsoft Edge, Microsoft’s replacement for Internet Explorer. You may consider using its `IE mode <https://docs.microsoft.com/en-us/deployedge/edge-ie-mode>`_ to keep access to IE11-only sites, while other sites and apps like Wagtail can leverage modern browser capabilities.
### Hallo legacy rich text editor has moved to an external package
* Hallo was deprecated in [Wagtail v2.0 (February 2018)](https://docs.wagtail.org/en/stable/releases/2.0.html#new-rich-text-editor) and has had only a minimal level of support since then.
* If you still require Hallo for your Wagtail installation, you will need to install the [Wagtail Hallo editor](https://github.com/wagtail/wagtail-hallo) legacy package.
* We encourage all users of the Hallo editor to take steps to migrate to the new Draftail editor as this external package is unlikely to have ongoing maintenance.
*`window.registerHalloPlugin` will no longer be created on the page editor load, unless the legacy package is installed.
As of this release, the use of special-purpose field panel types such as `StreamFieldPanel` and `ImageChooserPanel` is being phased out, and developers will generally expect to be able to use a plain `FieldPanel` instead. For this reason, developers of third-party packages implementing their own field panel types are recommended to follow suit and ensure that their code also works with `FieldPanel`. The steps for doing this will depend on the package's functionality, but in general:
* If the panel sets a custom template, your code should instead define [a `Widget` class](https://docs.djangoproject.com/en/stable/ref/forms/widgets/) that produces your desired HTML rendering.
* If the panel provides a `widget_overrides` method, your code should instead call [`register_form_field_override`](/extending/forms) so that the desired widget is always selected for the relevant model field type.
* If the panel provides a `get_comparison_class` method, your code should instead call `wagtail.admin.compare.register_comparison_class` to register the comparison class against the relevant model field type.
If you do continue to use a custom panel class, note that the template context for panels derived from `BaseChooserPanel` has changed - the context variable `is_chosen`, and the variable name given by the panel's `object_type_name` property, are no longer available on the template. The only available variables are now `field` and `show_add_comment_button`. If your template depends on these additional variables, you will need to pass them explicitly by overriding the `render_as_field` method.
### ModelAdmin forms must subclass `WagtailAdminModelForm`
When overriding the `get_form_class` method of a ModelAdmin `CreateView` or `EditView` to pass a custom form class, that form class must now inherit from `wagtail.admin.forms.models.WagtailAdminModelForm`. Passing a plain Django ModelForm subclass is no longer valid.
### Removed the `size` argument of the undocumented `wagtail.utils.sendfile_streaming_backend.was_modified_since` function
- The `size` argument was used to add a `length` parameter to the HTTP header.
- This was never part of the HTTP/1.0 and HTTP/1.1 specifications see [RFC7232](https://httpwg.org/specs/rfc7232.html#header.if-modified-since) and existed only as a an unofficial implementation in IE browsers.
