[web] Add dynamic view sizing (v2) (flutter#50271)

### Changes

* Introduces a new `viewConstraints` JS configuration parameter to configure max/min width/height constraints for a view. Those can have the following values:
  * An integer `>= 0`: max/min size in pixels
  * `Infinity` (or `Number.POSITIVE_INFINITY`): (only for max values) -> **unconstrained**.
  * When any value is not set, it defaults to "tight to the current size".
    * See [Understanding constraints](https://docs.flutter.dev/ui/layout/constraints).
* Computes the correct `physicalConstraints` of a view off of its `physicalSize` and its `viewConstraints` for the framework to use during layout.
  * When no constraints are passed, the current behavior is preserved: the default constraints are "tight" to the `physicalSize`.
* Resizes the current view DOM when requested by the framework and updates its internal physicalSize, then continues with the render procedure.

### Example

This is how we can configure a view to "take as much vertical space as needed":

```js
flutterApp.addView({
  viewConstraints: {
    minHeight: 0,
    maxHeight: Infinity,
  },
  hostElement: ...,
});
```

### TODO

* Needs actual unit tests

### Issues

* Fixes flutter/flutter#137444
* Closes flutter#48541

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

lib/web_ui/lib/platform_dispatcher.dart

@@ -90,8 +90,6 @@ abstract class PlatformDispatcher {
 
   void scheduleFrame();
 
-  Future<void> render(Scene scene, [FlutterView view]);
-
   AccessibilityFeatures get accessibilityFeatures;
 
   VoidCallback? get onAccessibilityFeaturesChanged;

lib/web_ui/lib/src/engine/js_interop/js_app.dart

@@ -25,9 +25,38 @@ extension JsFlutterViewOptionsExtension on JsFlutterViewOptions {
     return _hostElement!;
   }
 
+  @JS('viewConstraints')
+  external JsViewConstraints? get _viewConstraints;
+  JsViewConstraints? get viewConstraints {
+    return _viewConstraints;
+  }
+
   external JSAny? get initialData;
 }
 
+/// The JS bindings for a [ViewConstraints] object.
+@JS()
+@anonymous
+@staticInterop
+class JsViewConstraints {
+  external factory JsViewConstraints({
+    double? minWidth,
+    double? maxWidth,
+    double? minHeight,
+    double? maxHeight,
+  });
+}
+
+/// The attributes of a [JsViewConstraints] object.
+///
+/// These attributes are expressed in *logical* pixels.
+extension JsViewConstraintsExtension on JsViewConstraints {
+  external double? get maxHeight;
+  external double? get maxWidth;
+  external double? get minHeight;
+  external double? get minWidth;
+}
+
 /// The public JS API of a running Flutter Web App.
 @JS()
 @anonymous

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -797,27 +797,25 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   ///    scheduling of frames.
   ///  * [RendererBinding], the Flutter framework class which manages layout and
   ///    painting.
-  @override
   Future<void> render(ui.Scene scene, [ui.FlutterView? view]) async {
-    assert(view != null || implicitView != null,
-        'Calling render without a FlutterView');
-    if (view == null && implicitView == null) {
+    final EngineFlutterView? target = (view ?? implicitView) as EngineFlutterView?;
+    assert(target != null, 'Calling render without a FlutterView');
+    if (target == null) {
       // If there is no view to render into, then this is a no-op.
       return;
     }
-    final ui.FlutterView viewToRender = view ?? implicitView!;
 
     // Only render in an `onDrawFrame` or `onBeginFrame` scope. This is checked
     // by checking if the `_viewsRenderedInCurrentFrame` is non-null and this
     // view hasn't been rendered already in this scope.
     final bool shouldRender =
-        _viewsRenderedInCurrentFrame?.add(viewToRender) ?? false;
+        _viewsRenderedInCurrentFrame?.add(target) ?? false;
     // TODO(harryterkelsen): HTML renderer needs to violate the render rule in
     // order to perform golden tests in Flutter framework because on the HTML
     // renderer, golden tests render to DOM and then take a browser screenshot,
     // https://github.com/flutter/flutter/issues/137073.
     if (shouldRender || renderer.rendererTag == 'html') {
-      await renderer.renderScene(scene, viewToRender);
+      await renderer.renderScene(scene, target);
     }
   }
 

lib/web_ui/lib/src/engine/view_embedder/flutter_view_manager.dart

@@ -39,8 +39,11 @@ class FlutterViewManager {
   EngineFlutterView createAndRegisterView(
     JsFlutterViewOptions jsViewOptions,
   ) {
-    final EngineFlutterView view =
-        EngineFlutterView(_dispatcher, jsViewOptions.hostElement);
+    final EngineFlutterView view = EngineFlutterView(
+      _dispatcher,
+      jsViewOptions.hostElement,
+      viewConstraints: jsViewOptions.viewConstraints,
+    );
     registerView(view, jsViewOptions: jsViewOptions);
     return view;
   }

lib/web_ui/lib/src/engine/window.dart

@@ -15,6 +15,7 @@ import 'configuration.dart';
 import 'display.dart';
 import 'dom.dart';
 import 'initialization.dart';
+import 'js_interop/js_app.dart';
 import 'mouse/context_menu.dart';
 import 'mouse/cursor.dart';
 import 'navigation/history.dart';
@@ -50,7 +51,9 @@ base class EngineFlutterView implements ui.FlutterView {
   /// the Flutter view will be rendered.
   factory EngineFlutterView(
     EnginePlatformDispatcher platformDispatcher,
-    DomElement hostElement,
+    DomElement hostElement, {
+      JsViewConstraints? viewConstraints,
+    }
   ) = _EngineFlutterViewImpl;
 
   EngineFlutterView._(
@@ -59,8 +62,11 @@ base class EngineFlutterView implements ui.FlutterView {
     // This is nullable to accommodate the legacy `EngineFlutterWindow`. In
     // multi-view mode, the host element is required for each view (as reflected
     // by the public `EngineFlutterView` constructor).
-    DomElement? hostElement,
-  )   : embeddingStrategy = EmbeddingStrategy.create(hostElement: hostElement),
+    DomElement? hostElement, {
+      JsViewConstraints? viewConstraints,
+    }
+  )   : _jsViewConstraints = viewConstraints,
+        embeddingStrategy = EmbeddingStrategy.create(hostElement: hostElement),
         dimensionsProvider = DimensionsProvider.create(hostElement: hostElement) {
     // The embeddingStrategy will take care of cleaning up the rootElement on
     // hot restart.
@@ -117,7 +123,9 @@ base class EngineFlutterView implements ui.FlutterView {
   @override
   void render(ui.Scene scene, {ui.Size? size}) {
     assert(!isDisposed, 'Trying to render a disposed EngineFlutterView.');
-    // TODO(goderbauer): Respect the provided size when "physicalConstraints" are not always tight. See TODO on "physicalConstraints".
+    if (size != null) {
+      resize(size);
+    }
     platformDispatcher.render(scene, this);
   }
 
@@ -145,9 +153,14 @@ base class EngineFlutterView implements ui.FlutterView {
 
   late final PointerBinding pointerBinding;
 
-  // TODO(goderbauer): Provide API to configure constraints. See also TODO in "render".
   @override
-  ViewConstraints get physicalConstraints => ViewConstraints.tight(physicalSize);
+  ViewConstraints get physicalConstraints {
+    final double dpr = devicePixelRatio;
+    final ui.Size currentLogicalSize = physicalSize / dpr;
+    return ViewConstraints.fromJs(_jsViewConstraints, currentLogicalSize) * dpr;
+  }
+
+  final JsViewConstraints? _jsViewConstraints;
 
   late final EngineSemanticsOwner semantics = EngineSemanticsOwner(dom.semanticsHost);
 
@@ -156,6 +169,54 @@ base class EngineFlutterView implements ui.FlutterView {
     return _physicalSize ??= _computePhysicalSize();
   }
 
+  /// Resizes the `rootElement` to `newPhysicalSize` by changing its CSS style.
+  ///
+  /// This is used by the [render] method, when the framework sends new dimensions
+  /// for the current Flutter View.
+  ///
+  /// Dimensions from the framework are constrained by the [physicalConstraints]
+  /// that can be configured by the user when adding a view to the app.
+  ///
+  /// In practice, this method changes the size of the `rootElement` of the app
+  /// so it can push/shrink inside its `hostElement`. That way, a Flutter app
+  /// can change the layout of the container page.
+  ///
+  /// ```
+  /// <p>Some HTML content...</p>
+  /// +--- (div) hostElement ------------------------------------+
+  /// | +--- rootElement ---------------------+                  |
+  /// | |                                     |                  |
+  /// | |                                     |    container     |
+  /// | |    size applied to *this*           |    must be able  |
+  /// | |                                     |    to reflow     |
+  /// | |                                     |                  |
+  /// | +-------------------------------------+                  |
+  /// +----------------------------------------------------------+
+  /// <p>More HTML content...</p>
+  /// ```
+  ///
+  /// The `hostElement` needs to be styled in a way that allows its size to flow
+  /// with its contents. Things like `max-height: 100px; overflow: hidden` will
+  /// work as expected (by hiding the overflowing part of the flutter app), but
+  /// if in that case flutter is not made aware of that max-height with
+  /// `physicalConstraints`, it will end up rendering more pixels that are visible
+  /// on the screen, with a possible hit to performance.
+  ///
+  /// TL;DR: The `viewConstraints` of a Flutter view, must take into consideration
+  /// the CSS box-model restrictions imposed on its `hostElement` (especially when
+  /// hiding `overflow`). Flutter does not attempt to interpret the styles of
+  /// `hostElement` to compute its `physicalConstraints`, only its current size.
+  void resize(ui.Size newPhysicalSize) {
+    // The browser uses CSS, and CSS operates in logical sizes.
+    final ui.Size logicalSize = newPhysicalSize / devicePixelRatio;
+    dom.rootElement.style
+      ..width = '${logicalSize.width}px'
+      ..height = '${logicalSize.height}px';
+
+    // Force an update of the physicalSize so it's ready for the renderer.
+    _computePhysicalSize();
+  }
+
   /// Lazily populated and cleared at the end of the frame.
   ui.Size? _physicalSize;
 
@@ -278,8 +339,10 @@ base class EngineFlutterView implements ui.FlutterView {
 final class _EngineFlutterViewImpl extends EngineFlutterView {
   _EngineFlutterViewImpl(
     EnginePlatformDispatcher platformDispatcher,
-    DomElement hostElement,
-  ) : super._(_nextViewId++, platformDispatcher, hostElement);
+    DomElement hostElement, {
+      JsViewConstraints? viewConstraints,
+    }
+  ) : super._(_nextViewId++, platformDispatcher, hostElement, viewConstraints: viewConstraints);
 }
 
 /// The Web implementation of [ui.SingletonFlutterWindow].
@@ -708,6 +771,27 @@ class ViewConstraints implements ui.ViewConstraints {
       minHeight = size.height,
       maxHeight = size.height;
 
+  /// Converts JsViewConstraints into ViewConstraints.
+  ///
+  /// Since JsViewConstraints are expressed by the user, in logical pixels, this
+  /// conversion uses logical pixels for the current size as well.
+  ///
+  /// The resulting ViewConstraints object will be multiplied by devicePixelRatio
+  /// later to compute the physicalViewConstraints, which is what the framework
+  /// uses.
+  factory ViewConstraints.fromJs(
+    JsViewConstraints? constraints, ui.Size currentLogicalSize) {
+    if (constraints == null) {
+      return ViewConstraints.tight(currentLogicalSize);
+    }
+    return ViewConstraints(
+      minWidth: _computeMinConstraintValue(constraints.minWidth, currentLogicalSize.width),
+      minHeight: _computeMinConstraintValue(constraints.minHeight, currentLogicalSize.height),
+      maxWidth: _computeMaxConstraintValue(constraints.maxWidth, currentLogicalSize.width),
+      maxHeight: _computeMaxConstraintValue(constraints.maxHeight, currentLogicalSize.height),
+    );
+  }
+
   @override
   final double minWidth;
   @override
@@ -726,6 +810,15 @@ class ViewConstraints implements ui.ViewConstraints {
   @override
   bool get isTight => minWidth >= maxWidth && minHeight >= maxHeight;
 
+  ViewConstraints operator*(double factor) {
+    return ViewConstraints(
+      minWidth: minWidth * factor,
+      maxWidth: maxWidth * factor,
+      minHeight: minHeight * factor,
+      maxHeight: maxHeight * factor,
+    );
+  }
+
   @override
   ViewConstraints operator/(double factor) {
     return ViewConstraints(
@@ -774,3 +867,31 @@ class ViewConstraints implements ui.ViewConstraints {
     return 'ViewConstraints($width, $height)';
   }
 }
+
+// Computes the "min" value for a constraint that takes into account user `desired`
+// configuration and the actual available value.
+//
+// Returns the `desired` value unless it is `null`, in which case it returns the
+// `available` value.
+double _computeMinConstraintValue(double? desired, double available) {
+  assert(desired == null || desired >= 0, 'Minimum constraint must be >= 0 if set.');
+  assert(desired == null || desired.isFinite, 'Minimum constraint must be finite.');
+  return desired ?? available;
+}
+
+// Computes the "max" value for a constraint that takes into account user `desired`
+// configuration and the `available` size.
+//
+// Returns the `desired` value unless it is `null`, in which case it returns the
+// `available` value.
+//
+// A `desired` value of `Infinity` or `Number.POSITIVE_INFINITY` (from JS) means
+// "unconstrained".
+//
+// This method allows returning values larger than `available`, so the Flutter
+// app is able to stretch its container up to a certain value, without being
+// fully unconstrained.
+double _computeMaxConstraintValue(double? desired, double available) {
+  assert(desired == null || desired >= 0, 'Maximum constraint must be >= 0 if set.');
+  return desired ?? available;
+}

lib/web_ui/test/common/frame_timings_common.dart

@@ -5,36 +5,39 @@
 import 'dart:async';
 
 import 'package:test/test.dart';
+import 'package:ui/src/engine.dart' show EnginePlatformDispatcher;
 import 'package:ui/ui.dart' as ui;
 
 /// Tests frame timings in a renderer-agnostic way.
 ///
 /// See CanvasKit-specific and HTML-specific test files `frame_timings_test.dart`.
 Future<void> runFrameTimingsTest() async {
+  final EnginePlatformDispatcher dispatcher = ui.PlatformDispatcher.instance as EnginePlatformDispatcher;
+
   List<ui.FrameTiming>? timings;
-  ui.PlatformDispatcher.instance.onReportTimings = (List<ui.FrameTiming> data) {
+  dispatcher.onReportTimings = (List<ui.FrameTiming> data) {
     timings = data;
   };
   Completer<void> frameDone = Completer<void>();
-  ui.PlatformDispatcher.instance.onDrawFrame = () {
+  dispatcher.onDrawFrame = () {
     final ui.SceneBuilder sceneBuilder = ui.SceneBuilder();
     sceneBuilder
       ..pushOffset(0, 0)
       ..pop();
-    ui.PlatformDispatcher.instance.render(sceneBuilder.build()).then((_) {
+    dispatcher.render(sceneBuilder.build()).then((_) {
       frameDone.complete();
     });
   };
 
   // Frame 1.
-  ui.PlatformDispatcher.instance.scheduleFrame();
+  dispatcher.scheduleFrame();
   await frameDone.future;
   expect(timings, isNull, reason: "100 ms hasn't passed yet");
   await Future<void>.delayed(const Duration(milliseconds: 150));
 
   // Frame 2.
   frameDone = Completer<void>();
-  ui.PlatformDispatcher.instance.scheduleFrame();
+  dispatcher.scheduleFrame();
   await frameDone.future;
   expect(timings, hasLength(2), reason: '100 ms passed. 2 frames pumped.');
   for (final ui.FrameTiming timing in timings!) {