In brief
overflow-y controls vertical overflow. With a constrained height, overflow-y: auto lets content scroll inside a panel when it does not fit.
Useful to know
overflow-y controls vertical overflow. With a constrained height, overflow-y: auto lets content scroll inside a panel when it does not fit. [1]
Without a suitable constraint, the panel may simply grow. Before investigating a missing scrollbar, check its height and contents. Nested scrolling areas also need keyboard and touch checks.
Sources
For further detail and to check the information: