1. Go to "Tools" -> "Options..." -> "Build & Run" -> "General".
2. Click into the "Default build directory" edit.
3. Click on the appearing "AB" icon.
   The VariableChooser shows up.
4. Click on the "Kits" tab.
   The VariableChooser stays visible??
5. Select a kit.
6. Click into the kit's "Name" edit.
7. Have a guess where the variable will be inserted when you double-click in the VariableChooser. Don't cheat.

I think the VariableChooser should be closed when switching tabs.