In a generalized Difference-in-Differences in Dasgupta,2019 paper, he documented that
control countries did not have a leniency law introduced over 2 years before to 5 years after the introduction of the leniency law for the treated countries.
And it is how he describe sample selection
the treated group comprises all firms that are headquartered in countries that have passed a leniency law by year t. The control group comprises firms in countries that never adopted a leniency law in our sample period and firms headquartered in countries that adopted a leniency law at some later point of time
There is an example, Brazil implemented the law in 2000 and United States implemented the law in 1993. U.S implement the laws in 1993 means that Brazil in 1998 is control for US (instead of using firms in Brazil, I used Brazil here for shortening purposes). However, Brazil also passed the law in 2000, meaning that 1998 also belong to treatment sample.
So, what should we do in this case? In particular, which sample (control or treatment) that Brazil in 1998 belongs to?
Another example, Lithuania passed the law in 2008, Ukraine passed the law in 2017,so whether the firms in Lithuania from 2014 to 2017 is the control of Ukraine (because Lithuania from 2006->2013 are treatments). It makes sense if based on the switch on and off of Thomas from this discussion. However, on the other hand, it makes no sense, because the control for Ukraine cannot compared to the change of Lithuania before and after 2017. More detail about this example can be seen from the explanation here.