I'm not sure when it started - definitely quite recently - but my custom visual's drilldown has ceased to work on Desktop, Service and Mobile.
As I recall I didn't have to implement anything specific regarding selection to enable drilldown originally - the code that handled selection would just instigate drilldown when drill mode was turned on. Now though, even with drill mode turned on, clicking or tapping the selection just selects the data point as if drill mode was turned off.
I feel like this change might foreshadow the ability to trigger drilldown in other ways, which is something I requested a while back. If so, great, but for now I just need to fix my visual to re-enable the drilldown in the standard way. I will so searching in some of the core visual code if nobody can help, but first, any ideas?