Using HTML slots to render platform views in the web

Flutter now renders all web platform views in a consistent location of the DOM, as direct children of flt-glass-pane (regardless of the rendering backend: html or canvaskit). Platform views are then "slotted" into the correct position of the App's DOM with standard HTML features.

Up until this change, Flutter web would change the styling of the rendered contents of a platform views to position/size it to the available space. This is no longer the case. Users can now decide how they want to utilize the space allocated to their platform view by the framework.

The Flutter framework frequently tweaks its render tree to optimize the paint operations that are ultimately made per frame. In the web, these render tree changes often result in DOM operations.

Flutter web used to render its platform views (HtmlElementView widgets) directly into its corresponding position of the DOM.

Using certain DOM elements as the "target" of some DOM operations causes those elements to lose their internal state. In practice, this means that iframe tags are going to reload, video players might restart, or an editable form might lose its edits.

Flutter now renders platform views using slot elements inside of a single, app-wide shadow root. Slot elements can be added/removed/moved around the Shadow DOM without affecting the underlying slotted content (which is rendered in a constant location)

This change was made to:

• Stabilize the behavior of platform views in Flutter web.
• Unify how platform views are rendered in the web for both rendering backends (html and canvaskit).
• Provide a predictable location in the DOM that allows developers to reliably use CSS to style their platform views, and to use other standard DOM API, such as querySelector, and getElementById.

A Flutter web app is now rendered inside a common shadow root in which slot elements represent platform views. The actual content of each platform view is rendered as a sibling of said shadow root.

...
​
<flt-glass-pane>
  ...
  <div id="platform-view">Contents</div> <!-- canvaskit -->
  <!-- OR -->
  <flt-platform-view>
    #shadow-root
    | <div id="platform-view">Contents</div> <!-- html -->
  </flt-platform-view>
  ...
</flt-glass-pane>
​
...

...
​
<flt-glass-pane>
  #shadow-root
  | ...
  | <flt-platform-view-slot>
  |   <slot name="platform-view-1" />
  | </flt-platform-view-slot>
  | ...
  <flt-platform-view slot="platform-view-1">
    <div id="platform-view">Contents</div>
  </flt-platform-view>
  ...
</flt-glass-pane>
​
...

After this change, when the framework needs to move DOM nodes around, it operates over flt-platform-view-slots, which only contain a slot element. The slot projects the contents defined in flt-platform-view elements outside the shadow root. flt-platform-view elements are never the target of DOM operations from the framework, thus preventing the reload issues.

From an app's perspective, this change is transparent. However, this is considered a breaking change because some tests make assumptions about the internal DOM of a Flutter web app, and break.

The engine may print a warning message to the console similar to:

Height of Platform View type: [$viewType] may not be set. Defaulting to `height: 100%`.
Set `style.height` to any appropriate value to stop this message.

or:

Width of Platform View type: [$viewType] may not be set. Defaulting to `width: 100%`.
Set `style.width` to any appropriate value to stop this message.

Previously, the content returned by PlatformViewFactory functions was resized and positioned by the framework. Instead, Flutter now sizes and positions <flt-platform-view-slot>, which is the parent of the slot where the content is projected.

To stop the warning above, platform views need to set the style.width and style.height of their root element to any appropriate (non-null) value.

For example, to make the root html.Element fill all the available space allocated by the framework, set its style.width and style.height properties to '100%':

ui.platformViewRegistry.registerViewFactory(viewType, (int viewId) {
  final html.Element htmlElement = html.DivElement()
    // ..other props
    ..style.width = '100%'
    ..style.height = '100%';
  // ...
  return htmlElement;
});

If other techniques are used to lay out the platform view (like inset: 0) a value of auto for width and height is enough to stop the warning.

Read more about CSS width and CSS height.

After this change, user's test code does not need to deeply inspect the contents of the shadow root of the App. All of the platform view contents will be placed as direct children of flt-glass-pane, wrapped in a flt-platform-view element.

Avoid looking inside the flt-glass-pane shadow root, it is considered a "private implementation detail", and its markup can change at any time, without notice.

(See Relevant PRs below for examples of the "migrations" described above).

Landed in version: 2.3.0-16.0.pre
In stable release: 2.5

Design document:

• Using slot to embed web Platform Views

Relevant issues:

• Issue #80524

Relevant PRs:

• flutter/engine#25747: Introduces the feature.
• flutter/flutter#82926: Tweaks flutter tests.
• flutter/plugins#3964: Tweaks to plugins code.
• flutter/packages#364: Tweaks to packages code.