Why is svg.dispatch("input") necessary? I see the graphs are not in sync without it but don't understand why.

This cell is a view (viewof focus), and its value (focus) can be referenced reactively from other cells using Observable dataflow. The input event is used to tell Observable that the view’s value has changed. https://observablehq.com/@observablehq/introduction-to-views

Oh, I see. Thanks for the quick response. So basically this triggers the "update" block to run again with the new value on "focus".

Yep!

Compared with https://observablehq.com/d/953cd4edee39d6ec this version misses the zoom on the focus part. I'd like to make it work, because it's nice, but also in order to double-check that https://github.com/d3/d3-brush/pull/84 is correct.