Question

When using a QListWidget in batched layout mode, whenever more items are added than the batch size, the list widget blinks for a short time when switching from the old list to the new list. This means, the list widget shows no items, and the scroll bar handle is set to a seemingly random size.

Have you ever encountered this, can this be resolved somehow? I’m using Qt 4.7.4. I should probably add that I’m not using any hidden items.

Answer 1

I had this issue also and spent hours combing through the sea that is Qt widget rendering. Ultimately, like you, I traced the problem back to the batch processing of the QListView. It appears, that when batch processing is enabled, Qt fires off an internal timer to perform incremental layout adjustments of the underlying scroll view. During these incremental layouts, when the scroll bar is visible, the update region is not computed correctly (it’s too big and does not account for the regions occupied by the scroll widget(s) themselves). The result is a bad update region that subsequently finds its way into the viewport update which has the unfortunate side-effect of clearing the entire client area without rendering any of the ListViewItems.

Once the batch processing is complete, the final viewport update correctly computes the layout geometry (with the scroll bar) and produces a valid update region; the visible elements in the list are then redrawn.

The behavior worsens as the number of items in the list grows (relative to the batch size). For example, if your list grows from 500 to 50000 items and a batch size of 50, there is a proportionate increase in the number of “bad repaint” events which are triggered causing the view to visibly flicker even more. 🙁

These incremental (and failed) viewport updates also appear to be cause the apparent spazmodic behavior in the scrollbar handle position that you describe.

The root of this issue appears related to this “hack” that was added to
QListView::doItemsLayout() as commented here:

// showing the scroll bars will trigger a resize event,
// so we set the state to expanding to avoid
// triggering another layout
QAbstractItemView::State oldState = state();
setState(ExpandingState);

I suppose you could override QListView::doItemsLayout() and provide your own batch processing which handles scroll bars properly, but personally I’m too old and lazy to be cleaning up someone else’s poo. Switching to SinglePass eliminated the problem entirely. Seamless flicker-free rendering and the scroll bar behavior you’ve come to expect and love. Yay.