Fix drag selection stopping when scrolling beyond visible items

[Copilot]

Drag-to-select stops working when scrolling to the second page because gesture detectors were attached per-item in SliverChildBuilderDelegate, only receiving events for mounted items.

Changes

Moved gesture detection from per-item to scroll view level

• Wrapped CustomScrollView with a single GestureDetector handling all drag/pan/longpress events
• Removed per-item gesture detectors from sliverGrid builder (~75 lines)

Refactored coordinate calculation in AssetGridDragSelectionCoordinator

• Extracted _calculateIndexFromPosition() to compute grid indices from global screen coordinates
• Updated onSelectionStart() to accept BoxConstraints instead of index/asset parameters, calculating them from position
• Simplified onSelectionUpdate() to reuse shared position calculation (~80 lines of duplication removed)

How it works

// Before: Gesture detector on each item, limited to mounted items
child = GestureDetector(
  onHorizontalDragStart: (d) => coordinator.onSelectionStart(
    index: index,  // Only works for this specific mounted item
    asset: assets[index],
  ),
  child: child,
);

// After: Global gesture detector calculates index from position
GestureDetector(
  onHorizontalDragStart: (d) => coordinator.onSelectionStart(
    globalPosition: d.globalPosition,  // Works for any position
    constraints: constraints,
  ),
  child: CustomScrollView(...),
)

The position-to-index calculation accounts for grid parameters (count, spacing), scroll offset, reverted layouts, and special items, working independently of item mounting state.

Warning

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• https://storage.googleapis.com/flutter_infra_release/flutter/a5cb96369ef86c7e85abf5d662a1ca5d89775053/dart-sdk-linux-x64.zip
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• https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.24.5-stable.tar.xz
  • Triggering command: /usr/bin/wget wget -q REDACTED -O flutter.tar.xz (http block)
• https://storage.googleapis.com/flutter_infra_release/releases/stable/linux/flutter_linux_3.27.1-stable.tar.xz
  • Triggering command: /usr/bin/curl curl -L REDACTED --output flutter.tar.xz --progress-bar (http block)

If you need me to access, download, or install something from one of these locations, you can either:

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>[BUG] 拖拽选择到第二页会停止</issue_title>
<issue_description>Describe the bug
拖拽选择图片，滑动到第二页无法继续选择
https://github.com/user-attachments/assets/94d7241e-4987-4998-81a5-2b91d1ba80c0

How to reproduce
WeChat Assets Picker 示例

Version information

• WeChat Assets Picker: [v9.5.0]
• Flutter Version: [v3.24.1]
  </issue_description>

Comments on the Issue (you are @copilot in this section)

@AlexV525 This is limited by the current implementation of the auto-scroll gesture. It needs a refactor to determine scroll positions globally to get rid of the item mounting state. @AlexV525 > 感觉可以参考这个库的实现：[drag_select_grid_view](https://pub.dev/packages/drag_select_grid_view)

该项目相较于参考的库而言复杂度不在同一个等级，需要调研层级调整对 delegate 整体交互效果的影响。</comment_new>

• Fixes [BUG] 拖拽选择到第二页会停止 #695

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