This is a codeless post that will instead focus on a design issue present in all (at the time of writing) stable releases of Emacs. Be assured that you will not have to work around it in the upcoming Emacs 25 release.
Have you ever wondered why some commands deactivate the region afterwards, although there’s no explicit call to the deactivate-mark function? It turns out that this is intentional behavior as can be seen in the documentation of the deactivate-mark variable:
If an editing command sets this to t, deactivate the mark afterward. The command loop sets this to nil before each command, and tests the value when the command returns. Buffer modification stores t in this variable.
So, any command modifying a buffer will deactivate the region. Makes sense and if you for some reason need the region again, it’s a C-x C-x away. There is a major problem with this though, it doesn’t matter which buffer is modified…
This bit me hard with eyebrowse. I am using a modeline indicator to visualize its state which is using the built-in format-spec package. As that package is using a temporary buffer for turning a format string into a formatted string and the modeline indicator is recalculated very often, this led to the region being deactivated on any command. It took me quite a bit to figure this one out. I consider it madness for anyone to expect this behavior when writing functions that should not interfere with the region, so I’m glad it has been fixed in Emacs 25 by making the variable buffer-local.