QQuickItemView: Skip instantiating delegates if size is 0

If the area of a (List|Grid)View is 0, then instantiating delegates is
pointless, as they couldn't be shown anyway. However, our current logic
could not handle this case well, and would end up instantiating a
delegate for every delegate entry if their size also ended up being 0 -
you can after all fit infinitely many 0 sized items into a zero sized
container.
Detect this situation in QQuickItemViewPrivate::refill and the
applyInsertionChange implementations. Note that we only exit early if
there are no visible items and the view is zero-sized; if there are
visible items, we still want to ensure that they are removed after all.

We also need to adjust a few tests which had zero sized views to no
longer be zero sized; otherwise they wouldn't have created their
delegates in time.

[ChangeLog][QtQuick][ListView][Important Behavior Change] If a ListView
has size zero, it won't instantiate any delegates until its size becomes
non-zero.

Pick-to: 6.5
Fixes: QTBUG-110625
Fixes: QTBUG-89568
Fixes: QTBUG-51773
Change-Id: Ibe0e6fa5f01784016882522c120d2fee38df285b
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

1 files changed, 3 insertions, 0 deletions

diff --git a/src/quick/items/qquickgridview.cpp b/src/quick/items/qquickgridview.cpp
index 000f0ca887..2b0f70ab10 100644
--- a/src/quick/items/qquickgridview.cpp
+++ b/src/quick/items/qquickgridview.cpp
@@ -2358,6 +2358,9 @@ bool QQuickGridViewPrivate::applyInsertionChange(const QQmlChangeSet::Change &ch
 {
     Q_Q(QQuickGridView);
 
+    if (q->size().isEmpty())
+        return false;
+
     int modelIndex = change.index;
     int count = change.count;