Also see https://git.reviewboard.kde.org/r/103313/


QListView::updateGeometries() has it's own opinion on whether the scrollbars should be visible (valid range) or not
and triggers a (sometimes additionally timered) resize through ::layoutChildren()
http://qt.gitorious.org/qt/qt/blobs/4.7/src/gui/itemviews/qlistview.cpp#line1499
(the comment above the main block isn't all accurate, layoutChldren is called regardless of the policy)

As a result QListView and KCategorizedView occasionally started a race on the scrollbar visibility, effectively blocking the UI
So we prevent QListView from having an own opinion on the scrollbar visibility by
fixing it before calling the baseclass QListView::updateGeometries() and restoring the policy afterwards

Yes, resizing the "kcmshell4 kwincompositing", "all effects" KPluginSelector with large scrollbar sizes (bespin/position indicator/32px; oxygen was often sufficient with the default size) after commit e91e5fed6b1aad365e12e919f430c3e8147552d3 (see https://git.reviewboard.kde.org/r/103165/ ) was a reliable way to trigger the issue for me.
It also showed that resize/updateGeometries occured in pairs (ie 4calls forming  a block), what's never happened again with the patch.