On platforms such as XCB, the drag cursor pixmap is shown via a window
(a QShapedPixmapWindow) under the cursor.
The mouse button release event at the end of the drag is received in
this QXcbWindow, but intercepted by an event filter that QSimpleDrag
installs on the QApplication. It then resends it unmodified(!) after
the drag has ended and the drag pixmap window destroyed, causing it to
be delivered to the new top-level window.
The local coordinates in the unmodified QMouseEvent are local to the
drag pixmap window and don't match the window it is delayed-transmitted
to.
This ends up having fatal, user-visible effects particularly in Qt
Quick: QQuickWindow synthesizes a hover event once per frame using
the last received mouse coordinates, here: the release posted by
QSimpleDrag. This is done to update the hover event state for items
under the cursor when the mouse hasn't moved (e.g. QQuickMouseArea::
containsMouse). The bogus event coordinates in the release event then
usually end up causing an item near the top-left of the QQuickWindow
to assume it is hovered (because drag pixmap windows tend to be small),
even when the mouse cursor is actually far away from it at the end of
the drag.
This shows up e.g. in the Plasma 5 desktop, where dragging an icon
on the desktop will cause the icon at the top-left of the screen (if
any) to switch to hovered state, as the release coordinates on the
drag pixmap window (showing a dragged icon) fall into the geometry
of the top-left icon.
QSimpleDrag contains a topLevelAt() function to find the top-level
window under the global cursor coordinates that is not the drag
pixmap window. This is used by the drop event delivery code.
This patch uses this function to find the relevant top-level window,
then asks it to map the global cusor coordinates to its local
coordinate system, then synthesizes a new QMouseEvent with local
coordinates computed in this fashion. As a result the window now
gets a release event with coordinates that make sense and are
correct.
Task-number: QTBUG-66103
Change-Id: I04ebe6ccd4a991fdd4b540ff0227973ea8896a9d
Reviewed-by: Eike Hein <hein@kde.org>
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>
4a7771f206