Impeller engine: toImage() / capture produces truncated long screenshots (works fine on Skia)

[lzxyh0227]

When using the Impeller rendering engine, capturing a long image via RenderRepaintBoundary.toImage() (or similar capture methods) results in the right or bottom part of the image being cut off.
The same code works perfectly under Skia, producing a complete image.

`import 'package:flutter/material.dart';
import 'dart:ui' as ui;

class ScreenshotTestPage extends StatefulWidget {
const ScreenshotTestPage({super.key});

@OverRide
State createState() => _ScreenshotTestPageState();
}

class _ScreenshotTestPageState extends State {
final GlobalKey repaintKey = GlobalKey();

Future _capture() async {
final boundary =
repaintKey.currentContext!.findRenderObject() as RenderRepaintBoundary;
final ui.Image image = await boundary.toImage(pixelRatio: 3.0);
final byteData = await image.toByteData(format: ui.ImageByteFormat.png);
print('Captured image: ${image.width}x${image.height}, bytes: ${byteData?.lengthInBytes}');
}

@OverRide
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: const Text('Screenshot Test')),
body: SingleChildScrollView(
child: RepaintBoundary(
key: repaintKey,
child: Column(
children: List.generate(
100,
(i) => Container(
height: 100,
color: i.isEven ? Colors.blue : Colors.green,
child: Center(child: Text('Item $i')),
),
),
),
),
),
floatingActionButton: FloatingActionButton(
onPressed: _capture,
child: const Icon(Icons.camera_alt),
),
);
}
}`
The captured image should always match the widget’s full render area, regardless of whether Impeller or Skia is used.

Actual Behavior
• Under Impeller, the captured image is truncated (partially missing).
• Under Skia, the same code produces a complete image.

It seems that Impeller may create a render target or offscreen framebuffer limited to the visible viewport size, rather than the full logical size of the widget being captured.
As a result, when the target exceeds the current screen bounds (e.g., long scrollable views), part of the rendered content is clipped or discarded during capture.