The "I" key opens/closes a proper inventory window.
When open, the tab key '''removes only the "official" windows components(*). The remaining inventory window and all its contents are still shown.
(*) title and title area, widgets (transparency / lock / close), and "box" (that goes all around the rectangular part of the inventory window).
If you need pictures, let me know.
This action changed about a week or so ago, I believe. Previously, the action of the inventory