showing (or not) of folders in left pane

When I started using XP on he

In Windows Explorer, I have the left pane showing the tree of folders
(much as it was, by default I think, in Windows 9x).

If I opened a sub-folder by clicking on it in the _right_ pane, rather
than expanding in the left pane, the left pane showed _just_ the
sub-folder I'd opened - as if it was the only sub-folder which the
parent folder contained. If I entered other folders in that parent
folder, they also appeared in the left pane, but all the folders within
the parent folder were never shown in the left pane unless I explicitly
clicked on the [+] to expand it.

Now, if I enter any sub-folder in the right pane, that folder _and all
the others at the same level_ show in the left pane.

What have I (or a Microsoft update or ...?) changed?
--
J. P. Gilliver. UMRA: 1960/1985 MB++G.5AL-IS-P--Ch++(p)Ar@T0H+Sh0!:`)DNAf

.... more doomed than a busload of ... enthusiasts on their way to a Private
Frazer convention on a bus whose brakes have just failed as it heads towards a
cliff. - Eddie Mair, Radio Times 20-26 November 2010