is an option for cells that specifies an expression to be dynamically updated whenever the cell is visible on-screen.
- CellDynamicExpression is typically used with RuleDelayed.
- CellDynamicExpression->Dynamic[expr] causes the expression to be tracked and updated in the usual way, except that expr is not displayed anywhere.
- CellDynamicExpression may not trigger dynamic updates if the cell is scrolled off-screen or inside of a closed cell group.
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