[web] PointerBinding per view (#48248)

- Move initialization of `PointerBinding`/`KeyboardBinding` out of `FlutterViewEmbedder`.
- `computeEventOffsetToTarget` properly handles events within a given view.
- `PointerBinding` operates on the given Flutter view (it still listens to some `domWindow` events for the implicit view).
    - Stop using globals e.g. `ui.window`, `KeyboardBinding.instance`, `SafariPointerEventWorkaround.instance`, etc.
- `pointer_binding_test.dart` doesn't use globals either.
- `clickDebouncer` is now a static property on `PointerBinding`.

Fixes flutter/flutter#137289

lib/web_ui/lib/src/engine/embedder.dart

@@ -8,9 +8,7 @@ import '../engine.dart' show buildMode, renderer;
 import 'browser_detection.dart';
 import 'configuration.dart';
 import 'dom.dart';
-import 'keyboard_binding.dart';
 import 'platform_dispatcher.dart';
-import 'pointer_binding.dart';
 import 'text_editing/text_editing.dart';
 import 'view_embedder/style_manager.dart';
 import 'window.dart';
@@ -67,14 +65,6 @@ class FlutterViewEmbedder {
 
     renderer.reset(this);
 
-    // TODO(mdebbar): Move these to `engine/initialization.dart`.
-
-    KeyboardBinding.initInstance();
-    PointerBinding.initInstance(
-      _flutterViewElement,
-      KeyboardBinding.instance!.converter,
-    );
-
     window.onResize.listen(_metricsDidChange);
   }
 

lib/web_ui/lib/src/engine/initialization.dart

@@ -225,6 +225,8 @@ Future<void> initializeEngineUi() async {
   _initializationState = DebugEngineInitializationState.initializingUi;
 
   RawKeyboard.initialize(onMacOs: operatingSystem == OperatingSystem.macOs);
+  KeyboardBinding.initInstance();
+
   if (!configuration.multiViewEnabled) {
     ensureImplicitViewInitialized(hostElement: configuration.hostElement);
     ensureFlutterViewEmbedderInitialized();

lib/web_ui/lib/src/engine/keyboard_binding.dart

@@ -258,6 +258,7 @@ class KeyboardConverter {
   bool _disposed = false;
   void dispose() {
     _disposed = true;
+    clearPressedKeys();
   }
 
   // On macOS, CapsLock behaves differently in that, a keydown event occurs when
@@ -699,4 +700,8 @@ class KeyboardConverter {
   bool keyIsPressed(int physical) {
     return _pressingRecords.containsKey(physical);
   }
+
+  void clearPressedKeys() {
+    _pressingRecords.clear();
+  }
 }

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

@@ -634,7 +634,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         const StandardMessageCodec codec = StandardMessageCodec();
         // TODO(yjbanov): Dispatch the announcement to the correct view?
         //                https://github.com/flutter/flutter/issues/137445
-        implicitView!.accessibilityAnnouncements.handleMessage(codec, data);
+        implicitView?.accessibilityAnnouncements.handleMessage(codec, data);
         replyToPlatformMessage(callback, codec.encodeMessage(true));
         return;