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, 2 insertions, 0 deletions

diff --git a/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp b/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
index e522f280a3..6fd173fb0c 100644
--- a/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
+++ b/tests/auto/qml/qqmllistmodel/tst_qqmllistmodel.cpp
@@ -1549,6 +1549,8 @@ void tst_qqmllistmodel::modify_through_delegate()
         "       ListElement { name: \"Doe\"; age: 33 }\n"
         "   }\n"
         "   ListView {\n"
+        "       height: 100\n" \
+        "       width: 100\n" \
         "       model: testModel\n"
         "       delegate: Item {\n"
         "           Component.onCompleted: model.age = 18;\n"