fix: WindowDragArea dragging delays (#5795)

* improve `WindowDragArea` double-tap handling

* feat: `Offset.distance` property

* reorder Offset props

* improve webview docs

* improve webview docs

* Set Markdown value at initialization in tests

Assigns the 'value' property of the Markdown control during initialization instead of within each test. Also removes the 'similarity_threshold' parameter from screenshot assertions for consistency.

---------

Co-authored-by: Feodor Fitsner <feodor@appveyor.com>

Author: ndonkoHenri

packages/flet/lib/src/controls/window_drag_area.dart

@@ -1,61 +1,122 @@
+import 'dart:async';
+
 import 'package:flet/src/extensions/control.dart';
 import 'package:flet/src/utils/events.dart';
 import 'package:flet/src/utils/numbers.dart';
+import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 import 'package:window_manager/window_manager.dart';
 
 import '../models/control.dart';
 import '../widgets/error.dart';
 import 'base_controls.dart';
 
-class WindowDragAreaControl extends StatelessWidget {
+class WindowDragAreaControl extends StatefulWidget {
   final Control control;
 
   const WindowDragAreaControl({super.key, required this.control});
 
   @override
-  Widget build(BuildContext context) {
-    debugPrint("WindowDragArea build: ${control.id}");
+  State<WindowDragAreaControl> createState() => _WindowDragAreaControlState();
+}
+
+class _WindowDragAreaControlState extends State<WindowDragAreaControl> {
+  /// Timestamp of the last tap-up event, used for double-tap detection.
+  DateTime? _lastTapUpTime;
+
+  /// Position of the last tap-up event, used for double-tap detection.
+  Offset? _lastTapUpPosition;
+
+  /// Whether double-tap-to-maximize behavior is enabled.
+  bool get _maximizable => widget.control.getBool("maximizable", true)!;
+
+  /// Called when the user presses down inside the drag area.
+  ///
+  /// If a recent tap-up occurred close in time and space (within Flutter’s
+  /// [kDoubleTapTimeout] and [kDoubleTapSlop]), this is treated as a double-tap
+  /// and triggers a maximize/unmaximize toggle.
+  void _handlePointerDown(PointerDownEvent event) {
+    if (!_maximizable || _lastTapUpTime == null) return;
 
-    var content = control.buildWidget("content");
+    final now = DateTime.now();
+    final timeDiff = now.difference(_lastTapUpTime!);
+    final posDiff = (event.position - _lastTapUpPosition!).distance;
 
+    // If tap timing and distance match Flutter's double-tap thresholds
+    // — treat this as a double-tap.
+    if (timeDiff <= kDoubleTapTimeout && posDiff <= kDoubleTapSlop) {
+      _resetDoubleTapState();
+      unawaited(_toggleMaximize());
+    }
+  }
+
+  /// Called when the user lifts their finger or mouse button.
+  ///
+  /// Records the time and position so the next pointer down can detect
+  /// a double-tap sequence.
+  void _handlePointerUp(PointerUpEvent event) {
+    if (!_maximizable) return;
+    _lastTapUpTime = DateTime.now();
+    _lastTapUpPosition = event.position;
+  }
+
+  /// Clears any stored double-tap tracking information.
+  void _resetDoubleTapState() {
+    _lastTapUpTime = null;
+    _lastTapUpPosition = null;
+  }
+
+  /// Toggles between maximized and restored window states.
+  Future<void> _toggleMaximize() async {
+    final isMaximized = await windowManager.isMaximized();
+
+    if (isMaximized) {
+      await windowManager.unmaximize();
+    } else {
+      await windowManager.maximize();
+    }
+
+    widget.control
+        .triggerEvent("double_tap", isMaximized ? "unmaximize" : "maximize");
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    debugPrint("WindowDragArea build: ${widget.control.id}");
+
+    final content = widget.control.buildWidget("content");
     if (content == null) {
       return const ErrorControl(
           "WindowDragArea.content must be provided and visible");
     }
 
-    final wda = GestureDetector(
+    Widget dragArea = GestureDetector(
       behavior: HitTestBehavior.translucent,
       onPanStart: (DragStartDetails details) {
+        // Start moving the window.
         windowManager.startDragging();
-        if (control.getBool("on_drag_start", false)!) {
-          control.triggerEvent("drag_start", details.toMap());
-        }
+
+        widget.control.triggerEvent("drag_start", details.toMap());
       },
       onPanEnd: (DragEndDetails details) {
-        if (control.getBool("on_drag_end", false)!) {
-          control.triggerEvent("drag_end", details.toMap());
-        }
+        widget.control.triggerEvent("drag_end", details.toMap());
       },
-      onDoubleTap: control.getBool("maximizable", true)!
-          ? () async {
-              final isMaximized = await windowManager.isMaximized();
-              if (isMaximized) {
-                windowManager.unmaximize();
-              } else {
-                windowManager.maximize();
-              }
-
-              // trigger event
-              if (control.getBool("on_double_tap", false)!) {
-                control.triggerEvent(
-                    "double_tap", isMaximized ? "unmaximize" : "maximize");
-              }
-            }
-          : null,
       child: content,
     );
 
-    return LayoutControl(control: control, child: wda);
+    // If maximization is enabled, wrap with a listener to detect double-taps.
+    // Using a [Listener] instead of the above [GestureDetector] ensures the
+    // widget doesn’t block or consume gestures from its children.
+    if (_maximizable) {
+      dragArea = Listener(
+        behavior: HitTestBehavior.translucent,
+        onPointerDown: _handlePointerDown,
+        onPointerUp: _handlePointerUp,
+        onPointerCancel: (_) => _resetDoubleTapState(),
+        child: dragArea,
+      );
+    }
+
+    return LayoutControl(control: widget.control, child: dragArea);
   }
 }