When the user cancels the installation of an extension by pressing the cancel
button in the Extension Manager gui, then all files of an extension which have
already been processed (registered) need to be removed again.That's why the
progressbar does not stop immediately. Try this with the sunsearchtoolbar, and
you will understand. After pressing the  Cancel button, the text to the left of
the progress bar, which saids "Adding xyz", should be changed to, for example,
to "Removing". Then the user sees that clicking Cancel has some effect.

When the user removes an extension and presses cancel then the already
unregistered parts of the extension need to be registered again. Therefore the
progress bar will continue to be displayed for a while after clicking cancel.
The string displayed should then change from "Removing xyz" to , for example,
"Restoring xyz".

There is also the case when there is the same extension already installed. When
adding the extension, then the user must confirm the installation. If he agrees
the installed extension will be removed and thereafter the other one will be
installed. For the user this removing / installing is only one operation. During
that time the string "Adding xyz" is being displayed. Whe the user cancels the
installation then the previously installed extension will be restored again.
This should also be expressed by a suitable string.

In short, here are the three cases again:

1. Adding. The extension is not yet installed.

2. Adding. The same extension was already installed.

3. Removing.