fitGridColumnLeft function
The on-screen x of a column's leading edge, in the section's coordinates.
The assertion to make about a pinned column: it should not move when the grid is scrolled sideways.
Implementation
double fitGridColumnLeft(String columnId, {int index = 0}) {
final section = fitGridSection(index: index);
final at = section.columnLayout.indexOf(columnId);
if (at < 0) throw StateError('No visible column "$columnId"');
return section.debugColumnLeft(at);
}