The region container doesn’t get mouse events from the content. When you click on the region event never pops up to the region container. It stays on the content, like if it was not inside.
Created attachment 193236 [details]
This is a known issue that is not specific to regions but more to fragmentainers: how an event on the content inside a fragmentainer is passed on to the fragmentainer.
The best way to solve it is still under debate, there is an issue open on the regions spec on this: https://www.w3.org/Bugs/Public/show_bug.cgi?id=21132
This still seems to repro on today's nightly (r151543)