The member's own logical index its linked crosshair is marking, or null when it is showing none (it is the chart being hovered, the leader's instant falls outside its coverage, or crosshair sync is off).
A host wanting the linked bar's OHLC reads it back with
chart.dataLayer.indexToTime(...), the same way a native crosshair readout
does.
Flip switches at runtime. Turning crosshair off clears the linked lines
immediately rather than leaving the last one frozen on every follower;
turning symbol on makes the group agree on the instrument it already
knows, because a switch that only takes effect on the next change would
leave a linked grid visibly unlinked.
interval and chartType converge the same way, on the latest selection.
Turning drawings on joins every member that has an adapter, and turning
it off makes each leave.
viewport has no equivalent convergence: nothing in the group says which
member's window the others should have adopted, so it takes effect on the
next pan or zoom.
The host reporting an instrument change on one member: the imperative twin
of emitting 'symbol' on that chart's event bus. Records it and, when
symbol sync is on, loads it into every other member that can follow.
Put a chart in the group. Adding one twice updates its member options instead of double-subscribing.
A member joining a group that already has a symbol adopts it (when symbol sync is on and it can follow), because joining a linked workspace is exactly the moment a user expects the new chart to fall in line. The same holds for the interval and the chart type, and a member joining while drawings are shared joins the sharing.