Fix ComboBox single-item dropdown crash on open

[Copilot]

Opening a ComboBox with exactly one item could throw Invalid argument(s): 0.0 from clamp-bound calculations during menu clip/paint animation. This regressed behavior that previously worked and was triggered by single-item dropdown geometry.

• Root cause + guardrails in menu geometry

  • Updated clamp bound math in combo_box.dart to ensure lower/upper bounds are always valid when menu height is smaller than one item during transition frames.
  • Applied the same bound normalization in both:
    • _ComboBoxResizeClipper.getClip
    • _ComboBoxMenuPainter.paint
  • This keeps clip and paint paths consistent and prevents double.clamp from receiving inverted bounds.

• Regression coverage

  • Added a focused widget test in test/combo_box_test.dart that opens a one-item ComboBox and asserts no exception is produced.

• Changelog

  • Added a [next] entry documenting the single-item ComboBox crash fix.

final maxTopOffset = math.max(0.0, size.height - kComboBoxItemHeight);
final minBottomOffset = math.min(kComboBoxItemHeight, size.height);

final top = selectedItemOffset.clamp(0.0, maxTopOffset);
final bottom = (top + kComboBoxItemHeight).clamp(minBottomOffset, size.height);

• Fixes 🐛 Combobox with only one item raises error when clicked #1346