fix(windows): reverse horizontal scroll direction for synthetic wheel input

flutter_inappwebview_windows/CHANGELOG.md

@@ -1,5 +1,8 @@
 ## 0.7.0-beta.3
 
+- Fixed reversed horizontal scrolling: two-finger trackpad pans and mouse
+  tilt-wheel scrolls moved the content in the opposite direction, because
+  `WM_MOUSEHWHEEL`'s sign convention is inverted relative to `WM_MOUSEWHEEL`.
 - Fixed WebView2 input windows intercepting desktop clicks while the Flutter
   host window is minimized, including WebViews created or resumed during
   minimization.

flutter_inappwebview_windows/lib/src/in_app_webview/custom_platform_view.dart

@@ -634,7 +634,7 @@ class _CustomPlatformViewState extends State<CustomPlatformView>
                     _controller._setCursorPos(signal.localPosition);
                     _stopFling();
                     _sendScrollDelta(
-                      -signal.scrollDelta.dx,
+                      signal.scrollDelta.dx,
                       -signal.scrollDelta.dy,
                     );
                   } else if (signal is PointerScrollInertiaCancelEvent) {
@@ -728,7 +728,8 @@ class _CustomPlatformViewState extends State<CustomPlatformView>
     );
   }
 
-  /// Forwards scroll deltas immediately, preserving fractional remainders.
+  /// Forwards raw WM wheel data immediately, preserving fractional
+  /// remainders. WM sign asymmetry: positive dy = up, positive dx = right.
   void _sendScrollDelta(double dx, double dy) {
     _scrollRemainderX += dx;
     _scrollRemainderY += dy;
@@ -744,7 +745,9 @@ class _CustomPlatformViewState extends State<CustomPlatformView>
 
   void _sendTrackpadScrollDelta(double dx, double dy) {
     final delta = _dominantAxis(Offset(dx, dy));
-    _sendScrollDelta(delta.dx, delta.dy);
+    // Content follows the fingers: fingers right = scroll left = negative
+    // hwheel (WM_MOUSEHWHEEL positive means right, unlike the vertical axis).
+    _sendScrollDelta(-delta.dx, delta.dy);
   }
 
   /// Starts synthetic inertia after a fast lifted pan.

flutter_inappwebview_windows/test/custom_platform_view_scroll_test.dart

@@ -224,10 +224,31 @@ void main() {
       await tester.pump();
 
       // 0.5px × 1.2 × 1.5 = 0.9 units per update: whole units flush on the
-      // 2nd and 3rd updates.
+      // 2nd and 3rd updates. Fingers left → positive hwheel (scroll right is
+      // negative in WM terms only for the vertical axis).
       expect(scrollDeltaCalls(), [
-        [-1.0, 0.0],
-        [-1.0, 0.0],
+        [1.0, 0.0],
+        [1.0, 0.0],
+      ]);
+    });
+
+    testWidgets('horizontal pan follows the fingers (hwheel sign is inverted '
+        'relative to the vertical axis)', (tester) async {
+      final center = await pumpView(tester);
+
+      final pointer = TestPointer(1, PointerDeviceKind.trackpad);
+      await tester.sendEventToBinding(pointer.panZoomStart(center));
+      // Fingers move right 10px: content must follow them, i.e. scroll left,
+      // which is a NEGATIVE WM_MOUSEHWHEEL delta (positive hwheel = right).
+      await tester.sendEventToBinding(
+        pointer.panZoomUpdate(center, pan: const Offset(10, 0)),
+      );
+      await tester.sendEventToBinding(pointer.panZoomEnd());
+      await tester.pump();
+
+      // 10px × 1.2 × 1.5 = 18 units, negated for the horizontal axis.
+      expect(scrollDeltaCalls(), [
+        [-18.0, 0.0],
       ]);
     });
 
@@ -246,7 +267,7 @@ void main() {
       await tester.pump();
 
       expect(scrollDeltaCalls(), [
-        [-18.0, 0.0],
+        [18.0, 0.0],
         [0.0, -18.0],
       ]);
     });
@@ -435,6 +456,27 @@ void main() {
       ]);
     });
 
+    testWidgets('horizontal wheel deltas are forwarded unnegated '
+        '(WM_MOUSEHWHEEL positive = right, unlike the vertical axis)', (
+      tester,
+    ) async {
+      final center = await pumpView(tester);
+
+      final pointer = TestPointer(1, PointerDeviceKind.mouse);
+      await tester.sendEventToBinding(pointer.hover(center));
+      // The engine maps WM_MOUSEHWHEEL without negation, so reproducing the
+      // original native delta means forwarding dx as-is.
+      await tester.sendEventToBinding(pointer.scroll(const Offset(120, 0)));
+      await tester.pump();
+      await tester.sendEventToBinding(pointer.scroll(const Offset(-120, 0)));
+      await tester.pump();
+
+      expect(scrollDeltaCalls(), [
+        [120.0, 0.0],
+        [-120.0, 0.0],
+      ]);
+    });
+
     testWidgets('wheel deltas are still forwarded negated', (tester) async {
       final center = await pumpView(tester);