QQuickMultiPointTouchArea: ignore sub-pixel noise at the drag threshold

updateTouchData() detects a grab gesture once a touch point has moved more
than QStyleHints::startDragDistance(). tst_QQuickMultiPointTouchArea::mouseGestureStarted
enforces this with a test which expects a move of +dragThreshold not to start
a drag and a further +1 to do so

However, at fractional scale factors the computed delta can end up having
floating point rounding errors due to scaling round-trips between native
and device independent coordinates. This would cause a mouseGestureStarted
test failure when updateTouchData() decided that startDragDistance + (~1e-13)
was larger than startDragDistance and start a drag.

Fix this by replacing the operator > based comparison with one that checks
for floating point rounding errors using qFuzzyCompare() as well.

Task-number: QTBUG-146901
Change-Id: I8da3894cd2c922a59e0aec3a2a16f81b9d03e4bd
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

1 files changed, 11 insertions, 2 deletions

diff --git a/src/quick/items/qquickmultipointtoucharea.cpp b/src/quick/items/qquickmultipointtoucharea.cpp
index 1865409de7..d36eebebda 100644
--- a/src/quick/items/qquickmultipointtoucharea.cpp
+++ b/src/quick/items/qquickmultipointtoucharea.cpp
@@ -659,14 +659,23 @@ void QQuickMultiPointTouchArea::updateTouchData(QEvent *event, RemapEventPoints
         if (!_stealMouse /* !ignoring gesture*/) {
             bool offerGrab = false;
             const int dragThreshold = QGuiApplication::styleHints()->startDragDistance();
+
+            // overThreshold test predicate which does not consider deltas with floating
+            // point rounding residuals to be over the threshld. Required for test stability
+            // at fractional scale factors where scaling may introduce rounding errors.
+            const auto overThreshold = [dragThreshold](qreal delta) {
+                return qAbs(delta) > dragThreshold
+                        && !qFuzzyCompare(qAbs(delta), qreal(dragThreshold));
+            };
+
             for (const QEventPoint &p : std::as_const(touchPoints)) {
                 if (p.state() == QEventPoint::State::Released)
                     continue;
                 const QPointF currentPos = mapFromScene(p.scenePosition());
                 const QPointF startPos = mapFromScene(p.scenePressPosition());
-                if (qAbs(currentPos.x() - startPos.x()) > dragThreshold)
+                if (overThreshold(currentPos.x() - startPos.x()))
                     offerGrab = true;
-                else if (qAbs(currentPos.y() - startPos.y()) > dragThreshold)
+                else if (overThreshold(currentPos.y() - startPos.y()))
                     offerGrab = true;
                 if (offerGrab)
                     break;