Slider: don't prevent stealing until drag threshold is exceeded

If a Slider is placed in a Flickable (or ListView, etc.),
preventStealing means that you cannot flick the flickable by dragging
over the Slider.  It's better to postpone setting that until we
detect whether the user is really attempting to drag the slider,
because we don't want to rule out use cases such as vertical sliders
in a horizontal ListView or vice-versa, so that you drag in
one direction to move the slider and in another direction to flick.
Platform drag threshold is exposed in Settings (from QStyleHints).
(reverts/refines c9665f3e2d1eb12f1cc5a569c20f5d569fafde7f)

Change-Id: I82feda6028f6effa5b26ad575a1933e48e971453
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>

3 files changed, 18 insertions, 1 deletions

diff --git a/src/controls/Private/qquickcontrolsettings.cpp b/src/controls/Private/qquickcontrolsettings.cpp
index d89ffc1e..e1d0cabb 100644
--- a/src/controls/Private/qquickcontrolsettings.cpp
+++ b/src/controls/Private/qquickcontrolsettings.cpp
@@ -45,6 +45,8 @@
 #include <qqmlengine.h>
 #include <qdir.h>
 #include <QTouchDevice>
+#include <QGuiApplication>
+#include <QStyleHints>
 
 QT_BEGIN_NAMESPACE
 
@@ -175,5 +177,10 @@ qreal QQuickControlSettings::dpiScaleFactor() const
     return 1.0;
 }
 
+qreal QQuickControlSettings::dragThreshold() const
+{
+    return qApp->styleHints()->startDragDistance();
+}
+
 
 QT_END_NAMESPACE
diff --git a/src/controls/Private/qquickcontrolsettings_p.h b/src/controls/Private/qquickcontrolsettings_p.h
index 2b36e662..0687e209 100644
--- a/src/controls/Private/qquickcontrolsettings_p.h
+++ b/src/controls/Private/qquickcontrolsettings_p.h
@@ -56,6 +56,7 @@ class QQuickControlSettings : public QObject
     Q_PROPERTY(QString styleName READ styleName WRITE setStyleName NOTIFY styleNameChanged)
     Q_PROPERTY(QString stylePath READ stylePath WRITE setStylePath NOTIFY stylePathChanged)
     Q_PROPERTY(qreal dpiScaleFactor READ dpiScaleFactor CONSTANT)
+    Q_PROPERTY(qreal dragThreshold READ dragThreshold CONSTANT)
     Q_PROPERTY(bool hasTouchScreen READ hasTouchScreen CONSTANT)
 
 public:
@@ -70,6 +71,7 @@ public:
     void setStylePath(const QString &path);
 
     qreal dpiScaleFactor() const;
+    qreal dragThreshold() const;
 
     bool hasTouchScreen() const;
 
diff --git a/src/controls/Slider.qml b/src/controls/Slider.qml
index e2be07d8..c2869563 100644
--- a/src/controls/Slider.qml
+++ b/src/controls/Slider.qml
@@ -218,8 +218,9 @@ Control {
 
         anchors.fill: parent
         hoverEnabled: true
-        preventStealing: true
         property int clickOffset: 0
+        property real pressX: 0
+        property real pressY: 0
 
         function clamp ( val ) {
             return Math.max(range.positionAtMinimum, Math.min(range.positionAtMaximum, val))
@@ -229,6 +230,8 @@ Control {
             if (pressed && __horizontal) {
                 var pos = clamp (mouse.x + clickOffset - fakeHandle.width/2)
                 fakeHandle.x = pos
+                if (Math.abs(mouse.x - pressX) >= Settings.dragThreshold)
+                    preventStealing = true
             }
         }
 
@@ -236,6 +239,8 @@ Control {
             if (pressed && !__horizontal) {
                 var pos = clamp (mouse.y + clickOffset- fakeHandle.height/2)
                 fakeHandle.y = pos
+                if (Math.abs(mouse.y - pressY) >= Settings.dragThreshold)
+                    preventStealing = true
             }
         }
 
@@ -247,6 +252,8 @@ Control {
             if (fakeHandle.contains(Qt.point(point.x, point.y))) {
                 clickOffset = __horizontal ? fakeHandle.width/2 - point.x : fakeHandle.height/2 - point.y
             }
+            pressX = mouse.x
+            pressY = mouse.y
         }
 
         onReleased: {
@@ -255,6 +262,7 @@ Control {
             if (!slider.updateValueWhileDragging)
                 range.position = __horizontal ? fakeHandle.x : fakeHandle.y;
             clickOffset = 0
+            preventStealing = false
         }
     }