The bounded layout released the hour table to overflow:visible (wrapper =
the one scroll box) but left the header grid's internal overflowX:auto —
two unrelated horizontal scroll mechanisms that useSyncScroll (which only
pairs the grids' INTERNAL scrollers) cannot reconcile, so headers and
events drifted. Release the header grid the same way; both grids stay at
container width so minmax tracks resolve identically, and header cells
paint their own background for the sticky pass. Demo now carries a
12-resource bounded day-view repro harness.

Verified in-browser: one scroller, identical column offsets under combined
H+V scroll, opaque sticky headers.