Plash: tools for practical least privilege
Saving a file from Gnumeric: Here, the powerbox is accessed from the "File => Save As" menu item.
Importing an image file into an existing document in Inkscape: Here, the powerbox is accessed through the "File => Import" menu item. Note that you can choose to grant read-only access to the file: Inkscape doesn't need to write to the file if you're only importing it.
One limitation of the powerbox implementation is that you can't preview images in the file chooser dialog.
The interesting thing about these screenshots is that they are not very interesting! They just show opening or saving a file via a familiar file chooser dialog box.
The difference from usual is that the file chooser now grants the application the right to access the file in question. The file chooser is asking the user to make a security decision, although the user does not need to be aware of that.
Once you have chosen a file using the powerbox, you don't need to confirm the application's right to access it further. So selecting "File => Save" will save a document without any annoying "Are you sure?" boxes popping up.
In these examples, the windows belong to two different processes. The application runs in a sandboxed process, which gets granted access to the individual files that the user picks via the file chooser. The file chooser dialog is provided by the powerbox manager, which runs in a separate process. The powerbox manager has access to all the user's files and can delegate selected files to the application.
There are three visible differences from normal file chooser dialogs:
The powerbox manager happens to use Gtk to display the file chooser, but it could equally use any other widget set; it can be a different widget set to the application, as the XEmacs example shows.
Saving a file from XEmacs: Here, the powerbox has been opened by typing C-x C-s (in a buffer that doesn't have a filename yet) or by selecting the "File => Save As" menu item. Usually, typing C-x C-f (find-file) causes XEmacs to prompt for a filename via the minibuffer (using Tab for filename completion). Plash's XEmacs/powerbox integration (written in elisp) replaces the minibuffer-based prompt with a dialog box.