QQWindow: cancel touch->mouse synthesis when touch is ungrabbed

Previously, each time a new touchpoint is pressed, we would purposely
forget which touchpoint was acting as the mouse, as part of "starting
over" with event delivery.  Conceptually "starting over" ought to mean
as freshly as possible; but in practice, if a user was using one finger
to interact with some mouse-only Item, and then presses a second finger
(whether intentionally or not), (s)he doesn't want the first interaction
to immediately end.  The multi-finger DragHandler must be able to take
over the grab from the Item which already had the grab; but it uses
a passive grab in the meantime to track the movement, and normally
takes over the exclusive grab only when its preconditions are met:
the point has to move past the drag threshold.  So we can wait until
then to reset the touchMouseId.

The concrete use cases are: double-tapping a map is supposed to zoom
in, even if there is a MouseArea on top.  And, while dragging a Slider
inside a Flickable, you should be able to start dragging the Flickable
with a second finger.  In the first case the issue was that the
MouseArea could grab while handling the synth-event, thus setting
touchMouseId; then touchMouseId was immediately reset again while
handling the second touchpoint, so the second touchpoint would also be
offered as a synth-mouse event to various items.  But while fixing
that, we have to avoid this issue in the Slider-in-Flickable case:
when the first touch press is delivered, Flickable takes the exclusive
grab temporarily; after moving the touchpoint, the Slider's
DragHandler steals the exclusive grab.  Then we try to deliver the
second touchpoint press: at this time, we don't want touchMouseId to
be set, because we want to be able to deliver synth-mouse events for
the second point so that Flickable can grab that one.  So it must be
that when DragHandler steals the grab, we can reset touchMouseId,
because the only reason it was set was that Flickable had the grab.

This result is achieved by having QQuickItem::touchUngrabEvent()
call a new QQuickWindowPrivate::cancelTouchMouseSynthesis() function.
It was already a good idea to have such a function since we always
reset touchMouseId and touchMouseDevice at the same time.
Also modify the docs to remind users that when subclassing
QQuickItem and overriding mouseUngrabEvent() or touchUngrabEvent()
they should call the base class implementation, to avoid bypassing
this new functionality.

Fixes: QTBUG-70998
Change-Id: I02894971e9047d4fa7ac9d062d6714c9183a8058
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

0 files changed, 0 insertions, 0 deletions