There is path/file completion already in make parts of IDEA: inside html/jsp <a> elements, spring xml file references, etc.
Some differences between this "in-editor" completion and the one offered by the "path" field are:

• editor completes where there is only one match, without showing popup menu
• editor automatically completes shared prefixes
• first item in popup menu is preselected
• End/Home keys work
• editor uses icons for the items in the menu

I really like the "Path" field, however every time I try to use it I feel clumsy. Unifying completion behavior/UI between editor and this field would make this feature more intuitive.