Add SH_ItemView_ScrollMode style hint and use it in QMacStyle

On OS X, the default scrolling mode of item views should be
per pixel instead of per item. We enforce this through a new
style hint. On all other platforms, the behavior remains the
same.

It's still possible to override the style hint by using the
regular scroll mode setters. Any subsequent style change will
result in a no-op once the setters have been called and until
the properties are reset.

Some auto-tests had to be update to to take the new behavior
into account.

[ChangeLog][QtWidgets][Styles] Added SH_ItemView_ScrollMode
style hint.

[ChangeLog][QtWidgets][Item Views] Item views scroll per pixel
on OS X now.

[ChangeLog][QtWidgets][Item Views] QAbstractItemView::verticalScrollMode
and QAbstractItemView::horizontalScrollMode are now resettable.

Change-Id: I3f923275c99aa4389323b52fc1c5455fe71f8d73
Task-number: QTBUG-50102
Reviewed-by: Jake Petroules <jake.petroules@theqtcompany.com>

7 files changed, 49 insertions, 5 deletions

diff --git a/src/widgets/itemviews/qabstractitemview.cpp b/src/widgets/itemviews/qabstractitemview.cpp
index d98f61b990..9135d1a23c 100644
--- a/src/widgets/itemviews/qabstractitemview.cpp
+++ b/src/widgets/itemviews/qabstractitemview.cpp
@@ -108,7 +108,9 @@ QAbstractItemViewPrivate::QAbstractItemViewPrivate()
         currentIndexSet(false),
         wrapItemText(false),
         delayedPendingLayout(true),
-        moveCursorUpdatedView(false)
+        moveCursorUpdatedView(false),
+        verticalScrollModeSet(false),
+        horizontalScrollModeSet(false)
 {
     keyboardInputTime.invalidate();
 }
@@ -137,6 +139,9 @@ void QAbstractItemViewPrivate::init()
     viewport->setBackgroundRole(QPalette::Base);
 
     q->setAttribute(Qt::WA_InputMethodEnabled);
+
+    verticalScrollMode = static_cast<QAbstractItemView::ScrollMode>(q->style()->styleHint(QStyle::SH_ItemView_ScrollMode, 0, q, 0));
+    horizontalScrollMode = static_cast<QAbstractItemView::ScrollMode>(q->style()->styleHint(QStyle::SH_ItemView_ScrollMode, 0, q, 0));
 }
 
 void QAbstractItemViewPrivate::setHoverIndex(const QPersistentModelIndex &index)
@@ -1237,12 +1242,14 @@ QAbstractItemView::EditTriggers QAbstractItemView::editTriggers() const
     \brief how the view scrolls its contents in the vertical direction
 
     This property controls how the view scroll its contents vertically.
-    Scrolling can be done either per pixel or per item.
+    Scrolling can be done either per pixel or per item. Its default value
+    comes from the style via the QStyle::SH_ItemView_ScrollMode style hint.
 */
 
 void QAbstractItemView::setVerticalScrollMode(ScrollMode mode)
 {
     Q_D(QAbstractItemView);
+    d->verticalScrollModeSet = true;
     if (mode == d->verticalScrollMode)
         return;
     QModelIndex topLeft = indexAt(QPoint(0, 0));
@@ -1261,18 +1268,27 @@ QAbstractItemView::ScrollMode QAbstractItemView::verticalScrollMode() const
     return d->verticalScrollMode;
 }
 
+void QAbstractItemView::resetVerticalScrollMode()
+{
+    auto sm = static_cast<ScrollMode>(style()->styleHint(QStyle::SH_ItemView_ScrollMode, 0, this, 0));
+    setVerticalScrollMode(sm);
+    d_func()->verticalScrollModeSet = false;
+}
+
 /*!
     \since 4.2
     \property QAbstractItemView::horizontalScrollMode
     \brief how the view scrolls its contents in the horizontal direction
 
     This property controls how the view scroll its contents horizontally.
-    Scrolling can be done either per pixel or per item.
+    Scrolling can be done either per pixel or per item. Its default value
+    comes from the style via the QStyle::SH_ItemView_ScrollMode style hint.
 */
 
 void QAbstractItemView::setHorizontalScrollMode(ScrollMode mode)
 {
     Q_D(QAbstractItemView);
+    d->horizontalScrollModeSet = true;
     if (mode == d->horizontalScrollMode)
         return;
     d->horizontalScrollMode = mode;
@@ -1289,6 +1305,13 @@ QAbstractItemView::ScrollMode QAbstractItemView::horizontalScrollMode() const
     return d->horizontalScrollMode;
 }
 
+void QAbstractItemView::resetHorizontalScrollMode()
+{
+    auto sm = static_cast<ScrollMode>(style()->styleHint(QStyle::SH_ItemView_ScrollMode, 0, this, 0));
+    setHorizontalScrollMode(sm);
+    d_func()->horizontalScrollModeSet = false;
+}
+
 #ifndef QT_NO_DRAGANDDROP
 /*!
     \since 4.2
@@ -1627,6 +1650,10 @@ bool QAbstractItemView::event(QEvent *event)
         break;
     case QEvent::StyleChange:
         doItemsLayout();
+        if (!d->verticalScrollModeSet)
+            resetVerticalScrollMode();
+        if (!d->horizontalScrollModeSet)
+            resetHorizontalScrollMode();
         break;
     case QEvent::FocusOut:
         d->checkPersistentEditorFocus();
diff --git a/src/widgets/itemviews/qabstractitemview.h b/src/widgets/itemviews/qabstractitemview.h
index fb1d222676..b8d386f234 100644
--- a/src/widgets/itemviews/qabstractitemview.h
+++ b/src/widgets/itemviews/qabstractitemview.h
@@ -75,8 +75,8 @@ class Q_WIDGETS_EXPORT QAbstractItemView : public QAbstractScrollArea
     Q_PROPERTY(SelectionBehavior selectionBehavior READ selectionBehavior WRITE setSelectionBehavior)
     Q_PROPERTY(QSize iconSize READ iconSize WRITE setIconSize NOTIFY iconSizeChanged)
     Q_PROPERTY(Qt::TextElideMode textElideMode READ textElideMode WRITE setTextElideMode)
-    Q_PROPERTY(ScrollMode verticalScrollMode READ verticalScrollMode WRITE setVerticalScrollMode)
-    Q_PROPERTY(ScrollMode horizontalScrollMode READ horizontalScrollMode WRITE setHorizontalScrollMode)
+    Q_PROPERTY(ScrollMode verticalScrollMode READ verticalScrollMode WRITE setVerticalScrollMode RESET resetVerticalScrollMode)
+    Q_PROPERTY(ScrollMode horizontalScrollMode READ horizontalScrollMode WRITE setHorizontalScrollMode RESET resetHorizontalScrollMode)
 
 public:
     enum SelectionMode {
@@ -147,9 +147,11 @@ public:
 
     void setVerticalScrollMode(ScrollMode mode);
     ScrollMode verticalScrollMode() const;
+    void resetVerticalScrollMode();
 
     void setHorizontalScrollMode(ScrollMode mode);
     ScrollMode horizontalScrollMode() const;
+    void resetHorizontalScrollMode();
 
     void setAutoScroll(bool enable);
     bool hasAutoScroll() const;
diff --git a/src/widgets/itemviews/qabstractitemview_p.h b/src/widgets/itemviews/qabstractitemview_p.h
index 4ba24a2786..a3af79abcb 100644
--- a/src/widgets/itemviews/qabstractitemview_p.h
+++ b/src/widgets/itemviews/qabstractitemview_p.h
@@ -461,6 +461,10 @@ public:
     mutable bool delayedPendingLayout;
     bool moveCursorUpdatedView;
 
+    // Whether scroll mode has been explicitly set or its value come from SH_ItemView_ScrollMode
+    bool verticalScrollModeSet;
+    bool horizontalScrollModeSet;
+
 private:
     mutable QBasicTimer delayedLayout;
     mutable QBasicTimer fetchMoreTimer;
diff --git a/src/widgets/styles/qcommonstyle.cpp b/src/widgets/styles/qcommonstyle.cpp
index 17c2ea2ba8..c28a70e3f9 100644
--- a/src/widgets/styles/qcommonstyle.cpp
+++ b/src/widgets/styles/qcommonstyle.cpp
@@ -5215,6 +5215,9 @@ int QCommonStyle::styleHint(StyleHint sh, const QStyleOption *opt, const QWidget
     case SH_Splitter_OpaqueResize:
         ret = true;
         break;
+    case SH_ItemView_ScrollMode:
+        ret = QAbstractItemView::ScrollPerItem;
+        break;
     default:
         ret = 0;
         break;
diff --git a/src/widgets/styles/qmacstyle_mac.mm b/src/widgets/styles/qmacstyle_mac.mm
index 009cc1b1b0..74554f6d3d 100644
--- a/src/widgets/styles/qmacstyle_mac.mm
+++ b/src/widgets/styles/qmacstyle_mac.mm
@@ -3043,6 +3043,9 @@ int QMacStyle::styleHint(StyleHint sh, const QStyleOption *opt, const QWidget *w
             ret = [NSScroller preferredScrollerStyle] == NSScrollerStyleOverlay;
         }
         break;
+    case SH_ItemView_ScrollMode:
+        ret = QAbstractItemView::ScrollPerPixel;
+        break;
     default:
         ret = QCommonStyle::styleHint(sh, opt, w, hret);
         break;
diff --git a/src/widgets/styles/qstyle.cpp b/src/widgets/styles/qstyle.cpp
index 34d5b49ee1..26bb227e29 100644
--- a/src/widgets/styles/qstyle.cpp
+++ b/src/widgets/styles/qstyle.cpp
@@ -1978,6 +1978,10 @@ void QStyle::drawItemPixmap(QPainter *painter, const QRect &rect, int alignment,
            tab is changed while dragging over the tabbar, in milliseconds. This
            enum value has been introduced in Qt 5.4
 
+    \value SH_ItemView_ScrollMode The default vertical and horizontal scroll mode as specified
+           by the style. Can be overridden with QAbstractItemView::setVerticalScrollMode() and
+           QAbstractItemView::setHorizontalScrollMode(). This enum value has been introduced in Qt 5.7.
+
     \sa styleHint()
 */
 
diff --git a/src/widgets/styles/qstyle.h b/src/widgets/styles/qstyle.h
index 425acc8301..0f9c9379f5 100644
--- a/src/widgets/styles/qstyle.h
+++ b/src/widgets/styles/qstyle.h
@@ -732,6 +732,7 @@ public:
         SH_Menu_SubMenuSloppyCloseTimeout,
         SH_Menu_SubMenuResetWhenReenteringParent,
         SH_Menu_SubMenuDontStartSloppyOnLeave,
+        SH_ItemView_ScrollMode,
         // Add new style hint values here
 
         SH_CustomBase = 0xf0000000