This relates to Icon Switching (Issue 36518).
Since High Contrast (HC) icons will become a seperate theme, I created an
optimized 'images.zip' with all HC icons removed. The following two icons were
deleted as part of this:
ui/default_images/res/minus_sch.png
ui/default_images/res/plus_sch.png

After deleting these two icons, although I was using the 'Default' icon theme
(i.e. not HC), the  tree list view stopped functioning correctly. This can be
seen by accessing Tools>Options (and is used elsewhere in OOo). On clicking on
the Plus / Minus images to expand or collapse the tree, nothing happened. The
tree can still be expanded/ collapsed when clicking on the actual branch itself.
When the icons were replaced, normal behaviour resumed. (Note: to reproduce
this, remember to delete your imagecache after changing images.zip).

Interestingly, when deleting the 'minus.png' and 'plus.png' (which you'd assume
are essential), although the icons were missing the tree functioned correctly.
This is of course the desired behaviour: in the absence of an expected icon,
degrade gracefully.

The tree list should not have any dependency on these HC icons.

Regards,
Andrew