A solution for that is still being discussed. A couple approaches will probably be piloted.
My current workaround is to used the same filter in the flat list Person view. And use it to change the Active Person focus.
I also discovered that the Filtering can become VASTLY more efficient if you make a Filter that uses a list of Person IDs instead of the custom filter. This is most appropriate with a tiny set like the cousins example. (In a tree with 29K people, the search time went from 8.79 seconds to 1.66 seconds)
It is a bit involved… but worth the effort if you will be exploring in a huge tree:
copy the results from the filter to the clipboard, select all in the clipboard, use the context menu to create a Custom Filter from the clipboard list, edit the rule to copy the list, cancel the custom filter, clear the Charts Filter gramplet, paste the list into the ID field, select the “Use regular expressions”, Find
The list of 27 IDs for my test looked like:
^I000206$|^I000207$|^I000210$|^I000320$|^I000388$|^I000389$|^I000390$|^I000391$|^I000392$|^I001725$|^I001729$|^I001748$|^I001877$|^I012588$|^I015524$|^I017759$|^I017761$|^I022095$|^I023283$|^I023285$|^I023287$|^I033894$|^I033896$|^I033900$|^I033910$|^I033915$|^I033921$
(The colors were made more pale in Edit → Preferences → Colors, Names were made Bold, and the “Compact” Box outlines were changed to 0.5px hairlines instead of 2px in the View → Configure…)
