I've had this problem in all versions of Civ4. When I enable Right-Click menus, I will occasionally lose the popup detailed hover text. I have found that clicking both mouse buttons at the same time at a random place over the map will sometimes fix this.

Yes, it is indeed annoying. You can also restore hovering pop-ups by going into city screen and hovering over production icons.

This glitch has been reported since the beginning and unfortunately has never been fixed.
It becomes even worse when your are playing demonstration games and naming your units.

As there is said workaround, I think there should be a final solution as well - ASAP.

i've noticed it happens most often when the last unit moved was a worker. hope this helps them find it.

EW

I have been able to reproduce this bug regularly by doing the following:

I click on a unit to activate it.
I then move the mouse away.
I then hold ALT on the keyboard.
I then hover the mouse over the upgrade cost to see the total, and then click the Upgrade button.

Everytime I do this in Warlords, I lose the right click menu. If I then click both mouse buttons on some area of the map without a city or unit, the right click menu comes back.