Bug 25727 updated Select2 to the latest stable release (4.0.13). We found some glitches about the width (see bug 25727 comment 19 for instance).
It seems that the next 4.1 is going to fix it, but still beta.
Created attachment 108988 [details] [review]
Bug 26288: Update Select2 to 4.1.0
Not ready for integration but it is ready for testing if we want to be prepared when 4.1.0 is released