[two_dimensional_scrollables] optimizes tableview janks with >250k rows

[wangfeihang]

[two_dimensional_scrollables] optimizes tableview janks with >250k rows

Description:

This PR optimizes the scrolling jank issue of the TableView component when the number of rows exceeds 250,000.

Root Cause:

The _updateFirstAndLastVisibleCell method in RenderTableViewport uses linear for-loop traversal on _columnMetrics and _rowMetrics to locate the visible boundary cells: _firstNonPinnedRow, _lastNonPinnedRow, _firstNonPinnedColumn, and _lastNonPinnedColumn. When the number of rows/columns is extremely large (e.g., >250k rows), this linear traversal causes significant main-thread blocking and scrolling jank.

Solution:

Replace the linear for-loop with binary search algorithm to find the visible boundary cells (_firstNonPinnedRow, _lastNonPinnedRow, _firstNonPinnedColumn, _lastNonPinnedColumn). Binary search reduces the time complexity from O(n) to O(log n), effectively optimizing the scrolling jank issue under large data volumes.

Fixes: #138271

Video performance comparison

before:
https://github.com/user-attachments/assets/ca5b8821-4bdb-411f-bb2c-63998ac7c0d9

after:
https://github.com/user-attachments/assets/ebbf96d9-9e04-4ede-ae79-cd433885d3ab

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request introduces a significant performance optimization for TableView with a large number of rows by replacing a linear search with a binary search to find visible cells. The implementation is correct and includes relevant tests. I've provided a few suggestions to simplify the new binary search helper function and its call sites for improved code clarity and to better reflect the underlying data invariants.

[gemini-code-assist]

The implementation of _binarySearchFirstFromMap can be simplified. The length parameter is redundant as it's always map.length. Additionally, the _rowMetrics and _columnMetrics maps are populated with contiguous integer keys starting from 0, making them dense. Therefore, the span == null check is unnecessary and can be removed, which also allows for using a non-nullable _Span type for span. This improves code clarity by reflecting the data structure's invariants. I'll suggest updates to the call sites in subsequent comments.

  int? _binarySearchFirstFromMap(Map<int, _Span> map, bool Function(_Span) condition) {
    if (map.isEmpty) {
      return null;
    }
    var low = 0;
    int high = map.length - 1;
    int? result;
    while (low <= high) {
      final int mid = low + ((high - low) >> 1);
      final _Span span = map[mid]!;
      if (condition(span)) {
        result = mid;
        high = mid - 1;
      } else {
        low = mid + 1;
      }
    }
    return result;
  }

[gemini-code-assist]

Updating this call to _binarySearchFirstFromMap to use the simplified signature, removing the redundant length parameter.

    _firstNonPinnedColumn = _binarySearchFirstFromMap(
      _columnMetrics,
      (span) => !span.isPinned && span.trailingOffset >= _targetLeadingColumnPixel,
    );

[gemini-code-assist]

Updating this call to _binarySearchFirstFromMap to use the simplified signature.

    _lastNonPinnedColumn = _binarySearchFirstFromMap(
      _columnMetrics,
      (span) => !span.isPinned && span.trailingOffset >= _targetTrailingColumnPixel,
    );

[gemini-code-assist]

Updating this call to _binarySearchFirstFromMap to use the simplified signature.

    _firstNonPinnedRow = _binarySearchFirstFromMap(
      _rowMetrics,
      (span) => !span.isPinned && span.trailingOffset >= _targetLeadingRowPixel,
    );

[gemini-code-assist]

Updating this call to _binarySearchFirstFromMap to use the simplified signature.

    _lastNonPinnedRow = _binarySearchFirstFromMap(
      _rowMetrics,
      (span) => !span.isPinned && span.trailingOffset >= _targetTrailingRowPixel,
    );