A. Consider a Horizon setup which knows about 2 Keystone endpoints (setting AVAILABLE_REGIONS, I'm refraining from using it because it'll change in future, see bug 1494251). And each of these Keystone endpoints has 2 service region within it, but these service regions a different, for example RegionOne and RegionTwo in Keystone1 and RegionNorth and RegionSouth in Keystone2. Currently last service region selected is stored in cookies, that means that if User first selects RegionSouth in Keystone2, then signs out and logs in into Keystone1 where he by default placed into RegionOne (effectively saving this new region in cookies), then, he returns back to Keystone2 his RegionSouth choice is lost.
B. Another specific setup with multi-endpoint Keystone is when within Keystone1 Region1 is the own Keystone1 cloud and Region2 are the resources of the Keystone2 own cloud, and for Keystone2 situation is the same - Region1 are foreign resources, Region2 are local ones. In that case most deployers would like to default to Region1 when logging into Keystone1 endpoint and default to Region2 when logging into Keystone2 endpoint.
The proposed solution is to
* make a default selection of a service region based on the endpoint User is logging in (fixes B)
* save last service region in a per-endpoint cookie (fixes A)
Please note that there also exists bug 1359774 which tries to remedy a different problem in a different way, not compatible with this one.