Listen to text spacing overrides on the web (flutter#178081)

Original PR/Discussion: flutter#172915

# Framework:
* `EditableText`/`SelectableText`, applies
`lineHeightScaleFactorOverride`, `wordSpacingOverride`, and
`letterSpacingOverride` to it's `TextStyle` similarly to how we already
do for bold platform overrides. Note `SelectableText` is built on
`EditableText` so it also applies these overrides.
* `Text`, applies `lineHeightScaleFactorOverride`,
`wordSpacingOverride`, and `letterSpacingOverride` to it's `TextStyle`
similarly to how we already do for bold platform overrides.
* Exposes line height override through
`MediaQueryData.lineHeightScaleFactorOverride` and
`maybeLineHeightScaleFactorOverrideOf(context)`.
* Exposes letter spacing override through
`MediaQueryData.letterSpacingOverride` and
`maybeLetterSpacingOverrideOf(context)`.
* Exposes word spacing override through
`MediaQueryData.wordSpacingOverride` and
`maybeWordSpacingOverrideOf(context)`.
* Exposes paragraph spacing override through
`MediaQueryData.paragraphSpacingOverride` and
`maybeParagraphSpacingOverrideOf(context)`.
* `MediaQuery.applyTextStyleOverrides()` \
`MediaQueryData.applyTextStyleOverrides()` to be able to reset/override
the text spacing settings on `MediaQueryData`.

# Engine:
* Introduces new members on `PlatformDispatcher` API that hold the text
spacing properties that are overridden on the web.
* We provide the `lineHeightScaleFactorOverride`,
`letterSpacingOverride`, `wordSpacingOverride`, and
`paragraphSpacingOverride` on the web by attaching a `ResizeObserver` to
an off-screen hidden element, when its size changes we capture its text
spacing CSS properties, and notify the framework through
`onMetricsChanged`.

Fixes flutter#142712


https://github.com/user-attachments/assets/aaaa3e74-c232-4956-acd2-ae1a4487e415

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

---------

Co-authored-by: Renzo Olivares <roliv@google.com>

Author: 2 people

engine/src/flutter/lib/ui/platform_dispatcher.dart

@@ -1121,6 +1121,59 @@ class PlatformDispatcher {
   /// This option is used by [showTimePicker].
   bool get alwaysUse24HourFormat => _configuration.alwaysUse24HourFormat;
 
+  /// The system-suggested height of the text, as a multiple of the font size.
+  ///
+  /// This value takes precedence over any text height specified at the
+  /// application level. For example, at framework level, in the [TextStyle]
+  /// for [Text], [SelectableText], and [EditableText] widgets, this value
+  /// overrides the existing value of [TextStyle.height] and [StrutStyle.height].
+  ///
+  /// Returns null when no override has been set by the system.
+  ///
+  /// If this value changes, [onMetricsChanged] will be called.
+  double? get lineHeightScaleFactorOverride => _configuration.lineHeightScaleFactorOverride;
+
+  /// The system-suggested amount of additional space (in logical pixels)
+  /// to add between each letter.
+  ///
+  /// A negative value can be used to bring the letters closer.
+  ///
+  /// This value takes precedence over any text letter spacing specified at the
+  /// application level. For example, at framework level, in the [TextStyle]
+  /// for [Text], [SelectableText], and [EditableText] widgets, this value
+  /// overrides the existing value of [TextStyle.letterSpacing].
+  ///
+  /// Returns null when no override has been set by the system.
+  ///
+  /// If this value changes, [onMetricsChanged] will be called.
+  double? get letterSpacingOverride => _configuration.letterSpacingOverride;
+
+  /// The system-suggested amount of additional space (in logical pixels)
+  /// to add between each sequence of white-space (i.e. between each word).
+  ///
+  /// A negative value can be used to bring the words closer.
+  ///
+  /// This value takes precedence over any text word spacing specified at the
+  /// application level. For example, at framework level, in the [TextStyle]
+  /// for [Text], [SelectableText], and [EditableText] widgets, this value
+  /// overrides the existing value of [TextStyle.wordSpacing].
+  ///
+  /// Returns null when no override has been set by the system.
+  ///
+  /// If this value changes, [onMetricsChanged] will be called.
+  double? get wordSpacingOverride => _configuration.wordSpacingOverride;
+
+  /// The system-suggested amount of additional space (in logical pixels)
+  /// to add following each paragraph in text.
+  ///
+  /// This value takes precedence over any text paragraph spacing specified at
+  /// the application level.
+  ///
+  /// Returns null when no override has been set by the system.
+  ///
+  /// If this value changes, [onMetricsChanged] will be called.
+  double? get paragraphSpacingOverride => _configuration.paragraphSpacingOverride;
+
   /// The system-reported text scale.
   ///
   /// This establishes the text scaling factor to use when rendering text,
@@ -1811,6 +1864,10 @@ class _PlatformConfiguration {
     this.defaultRouteName,
     this.systemFontFamily,
     this.configurationId,
+    this.lineHeightScaleFactorOverride,
+    this.letterSpacingOverride,
+    this.wordSpacingOverride,
+    this.paragraphSpacingOverride,
   });
 
   _PlatformConfiguration copyWith({
@@ -1834,6 +1891,10 @@ class _PlatformConfiguration {
       defaultRouteName: defaultRouteName ?? this.defaultRouteName,
       systemFontFamily: systemFontFamily ?? this.systemFontFamily,
       configurationId: configurationId ?? this.configurationId,
+      lineHeightScaleFactorOverride: lineHeightScaleFactorOverride,
+      letterSpacingOverride: letterSpacingOverride,
+      wordSpacingOverride: wordSpacingOverride,
+      paragraphSpacingOverride: paragraphSpacingOverride,
     );
   }
 
@@ -1881,6 +1942,25 @@ class _PlatformConfiguration {
   /// configuration updates from the embedder yet. The _getScaledFontSize
   /// function should not be called in either case.
   final int? configurationId;
+
+  /// The system-reported height of the text, as a multiple of the font size.
+  final double? lineHeightScaleFactorOverride;
+
+  /// The system-reported amount of additional space (in logical pixels)
+  /// to add between each letter.
+  ///
+  /// A negative value can be used to bring the letters closer.
+  final double? letterSpacingOverride;
+
+  /// The system-reported amount of additional space (in logical pixels)
+  /// to add between each sequence of white-space (i.e. between each word).
+  ///
+  /// A negative value can be used to bring the words closer.
+  final double? wordSpacingOverride;
+
+  /// The system-reported amount of additional space (in logical pixels)
+  /// to add between each paragraph in text.
+  final double? paragraphSpacingOverride;
 }
 
 /// An immutable view configuration.

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

@@ -89,6 +89,14 @@ abstract class PlatformDispatcher {
 
   void setApplicationLocale(Locale locale) {}
 
+  double? get lineHeightScaleFactorOverride;
+
+  double? get letterSpacingOverride;
+
+  double? get wordSpacingOverride;
+
+  double? get paragraphSpacingOverride;
+
   AccessibilityFeatures get accessibilityFeatures;
 
   VoidCallback? get onAccessibilityFeaturesChanged;