Expected behavior:
When you go through a portal, your selected item (wielded item, whatever) stays the same.
Observed behavior:
When you go through a portal, you always come out wielding whatever is in slot 1 of your hotbar (the leftmost slot).
Affected versions:
All that I've played.
This is pretty minor, and it may well be one of those things that would be way more difficult than it's worth, as far as software architecture goes. And for all I know it's been requested half a dozen times before, though I wasn't able to find any reports.
Thought I may as well put it out there since you have this shiny new issue tracker. 🙂
Duplicate of MC-1311