OnSnapshotChanged used to re-project every cluster group and re-render the
whole page for each driver publish, per open tab. It now only marks a
CoalescingDirtyFlag; the 5 s tick rebuilds when the flag is set or the config
load changed (or evicted) anything.
Refs #745
Claude-Session: https://claude.ai/code/session_01Ub9hGmYLQvn9XETzw7CMsT
Joseph Doherty