Handle the dragged item being destroyed during a native drag

Drag.dragType: Drag.Automatic goes through QDrag::exec(), which blocks in a
nested event loop. Anything happening inside that loop can destroy the item the
Drag attached object belongs to -- a ListView recycling the delegate that is
being dragged is enough, which is what auto-scrolling during a drag does. Both
the QQuickDragAttached and the QDrag are QObject children of that item, so they
are deleted with it, and every line after exec() in startDrag() then runs on
freed memory: the reported crash is in the dragFinished() emission.

~QQuickItem notifies its change listeners before ~QObject deletes its children,
so that is the last point at which the drag can still be wound down by an object
that is still alive. Listen for Destroyed for the duration of a native drag and
do it from there: cancel the platform drag, which unwinds the nested loop and
delivers the drag leave that the drop target is waiting for, then report the
drag as finished and inactive. Guard the QDrag and the attached object itself
across exec() for whatever gets there first.

Nothing here knows about item views, so it covers TableView, PathView, Repeater
and plain delegate models as well, and any route to the item's destruction
rather than only recycling. It does not try to keep the item alive; an
application that wants the drag to survive can still use
ListView.delayRemove: Drag.active.

The new test drives the nested event loop, which needs a QPlatformDrag that runs
in-process. It therefore runs on the offscreen platform, and skips if that
platform's drag implementation is a stub.

Fixes: QTBUG-124663
Pick-to: 6.12
Change-Id: I51c6719bcc9ea0e381a314f68f2f335caa96e616
Reviewed-by: Oliver Eftevaag <oliver.eftevaag@qt.io>

1 files changed, 1 insertions, 0 deletions

diff --git a/tests/auto/quick/CMakeLists.txt b/tests/auto/quick/CMakeLists.txt
index 035f960010..18ef46d300 100644
--- a/tests/auto/quick/CMakeLists.txt
+++ b/tests/auto/quick/CMakeLists.txt
@@ -43,6 +43,7 @@ if(QT_FEATURE_private_tests)
     endif()
     add_subdirectory(qquickdrag)
     add_subdirectory(qquickdragattached)
+    add_subdirectory(qquicknativedrag)
     add_subdirectory(qquickdroparea)
     add_subdirectory(qquickdynamicpropertyanimation)
     add_subdirectory(qquickflickable)