Fix ComboBox popup layout when opened at viewport bottom edge

[Copilot]

ComboBox could hit a debug layout assertion when the control was fully aligned to the bottom of the viewport, causing the popup to render with an invalid vertical position. This change hardens popup placement so the menu always stays within available bounds.

• Popup layout clamping

  • Updated _ComboBoxMenuRouteLayout.getPositionForChild to clamp the computed popup top based on the actual childSize.height and viewport height.
  • Replaced the previous assert assumptions (menuLimits.top + menuLimits.height <= size.height) with bounds checks on the clamped position used for rendering.

• Regression test for bottom-edge placement

  • Added a widget test that places ComboBox at the bottom of a fixed-height viewport, opens it, and asserts no exception is thrown while menu items render correctly.

• Changelog update

  • Added a 4.16.2 entry documenting the ComboBox bottom-edge assertion fix.

final top = menuLimits.top.clamp(
  0.0,
  math.max(0.0, size.height - childSize.height),
);

assert(top >= 0.0);
assert(top + childSize.height <= size.height);

return Offset(left, top);

• Fixes 🐛 ComboBox throws rendering exception when it is rendered at the bottom of the screen #1350