fix(flutter): correct cursor scaling and Linux cursor clipping

Keep Linux/macOS bitmap sizing and painted cursor DPI
conversion consistent, and derive hotspots from the actual scaled bitmap.

Pad tall Linux native cursor buffers with transparency to
preserve the lower half in the hardware cursor plane.
Cover bitmap sizing, hotspots, artwork preservation,
and painting with focused regression tests.

flutter/test/cursor_dpi_scale_test.dart

@@ -0,0 +1,111 @@
+import 'dart:io';
+
+import 'package:flutter/services.dart';
+import 'package:flutter_hbb/models/model.dart';
+import 'package:flutter_hbb/native/custom_cursor.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:image/image.dart' as img;
+
+// Source size, source hotspot, scale, artwork size, Linux hotspot.
+const _cases = [
+  ((9, 18), (4.0, 9.0), 1 / 3, (3, 6), (1.0, 3.0)),
+  ((17, 23), (4.0, 4.0), 1 / 3, (6, 8), (1.0, 1.0)),
+  ((32, 16), (16.0, 8.0), 0.5, (16, 8), (8.0, 4.0)),
+  ((24, 24), (11.0, 11.0), 1 / 3, (8, 8), (4.0, 4.0)),
+  ((3, 3), (2.0, 2.0), 1 / 3, (1, 1), (0.0, 0.0)),
+  ((1, 48), (0.0, 24.0), 1 / 3, (1, 16), (0.0, 8.0)),
+  ((9, 18), (4.0, 9.0), 1.0, (9, 18), (4.0, 9.0)),
+  ((9, 18), (4.0, 9.0), 7 / 18, (4, 7), (2.0, 4.0)),
+  ((24, 24), (4.0, 4.0), 0.1, (2, 2), (0.0, 0.0)),
+];
+const _hotspotTolerance = 1e-9;
+
+class _CursorModel implements CursorModel {
+  @override
+  final Set<String> cachedKeys = {};
+
+  @override
+  void addKey(String key) => cachedKeys.add(key);
+
+  @override
+  dynamic noSuchMethod(Invocation invocation) => super.noSuchMethod(invocation);
+}
+
+CursorData _cursorData((int, int) size, (double, double) hotspot) {
+  final image = img.Image(width: size.$1, height: size.$2, numChannels: 4);
+  for (var y = 0; y < image.height; y++) {
+    image.setPixelRgba(image.width ~/ 2, y, 255, 255, 255, 255);
+  }
+  for (var x = 0; x < image.width; x++) {
+    image.setPixelRgba(x, 0, 255, 0, 0, 255);
+    image.setPixelRgba(x, image.height - 1, 0, 0, 255, 255);
+  }
+  return CursorData(
+    peerId: 'cursor-test',
+    id: 'native',
+    image: image,
+    scale: 1,
+    data: Uint8List.fromList(img.encodePng(image)),
+    hotxOrigin: hotspot.$1,
+    hotyOrigin: hotspot.$2,
+    width: size.$1,
+    height: size.$2,
+  );
+}
+
+Future<Map<dynamic, dynamic>> _register(
+    WidgetTester tester, CursorData data, double scale) async {
+  const channel = MethodChannel('flutter_custom_cursor');
+  Map<dynamic, dynamic>? registered;
+  tester.binding.defaultBinaryMessenger.setMockMethodCallHandler(channel,
+      (call) async {
+    expect(call.method, 'createCustomCursor');
+    registered = call.arguments as Map<dynamic, dynamic>;
+    return registered!['name'];
+  });
+  addTearDown(() => tester.binding.defaultBinaryMessenger
+      .setMockMethodCallHandler(channel, null));
+  buildCursorOfCache(_CursorModel(), scale, data);
+  await tester.pump();
+  expect(registered, isNotNull);
+  return registered!;
+}
+
+void _expectArtwork(img.Image native, img.Image artwork) {
+  final content = img.copyCrop(native,
+      x: 0, y: 0, width: artwork.width, height: artwork.height);
+  expect(content.getBytes(), artwork.getBytes());
+  for (final pixel in native) {
+    if (pixel.x >= artwork.width) {
+      expect(pixel.a, 0, reason: 'Cursor padding must be transparent');
+    }
+  }
+}
+
+void main() {
+  for (final (source, hotspot, scale, size, linuxHotspot) in _cases) {
+    testWidgets('${source.$1}x${source.$2} cursor at scale $scale',
+        (tester) async {
+      final data = _cursorData(source, hotspot);
+      final cursor = await _register(tester, data, scale);
+      final artwork = img.decodePng(data.data!)!;
+      final native = img.decodePng(cursor['buffer'] as Uint8List)!;
+      final width = Platform.isLinux && size.$2 > size.$1 ? size.$2 : size.$1;
+
+      expect(data.scale, scale);
+      expect((artwork.width, artwork.height), size);
+      expect(data.hotx / size.$1,
+          closeTo(hotspot.$1 / source.$1, _hotspotTolerance));
+      expect(data.hoty / size.$2,
+          closeTo(hotspot.$2 / source.$2, _hotspotTolerance));
+      expect((native.width, native.height), (width, size.$2));
+      expect((cursor['width'], cursor['height']), (width, size.$2));
+      expect((cursor['hotX'], cursor['hotY']),
+          Platform.isLinux ? linuxHotspot : (data.hotx, data.hoty));
+      _expectArtwork(native, artwork);
+      if (width == artwork.width) {
+        expect(cursor['buffer'], data.data);
+      }
+    }, skip: Platform.isWindows);
+  }
+}