QAIV - allow users to control single step in ScrollPerPixel mode

The documentation says that we scroll one pixel, but changing the
behavior in 934f06220391eb0e0ebf66a2eb037f48adb4c43c to do that
was not well received.

People were relying on the undocumented behavior - and the new behavior
was considered to be a regression. (Nobody called setSingleStep since
Qt in many cases provide a reasonable singleStep - which implied that their
programs scrolled with 1 pixel which was quite slow). Furthermore getting
the old behavior (auto set of single step) was nearly impossible.

However the revert (done in 0e69230d02813f0b7a050645fb7e443bd504ab6a)
gets us back to QScrollbar::setSingleStep not working in pixel scroll mode
(even without it being documented - but we should also have a working API
rather than documenting that it is not working)

The previous approach was directly prevented Qt from changing
single step (on e.g resize) at all. This patch only prevents Qt from
changing when a user explicitly has called the function
QScrollBar::setSingleStep (in pixel scroll mode).

That is we expect that calls to setSingleStep means that the user actually
wants to set the singleStep and doesn't want Qt to control that value.

Furthermore it is possible to switch back to the automatically adjusted
singlestep with QScrollBar::setSingleStep(-1).

[ChangeLog][QtWidgets][QAbstractItemView] QTBUG-7232 - In ItemViews
when scrollMode is set to scrollPerPixel, it is now possible to
change the single step. Qt will automatically adjust the single step
until setSingleStep is called. When setSingleStep is called it will
however respect the set and stop doing automatic changes of the value.
Calling setSingleStep(-1) will switch mode back to automatic adjust.

Task-number: QTBUG-7232
Change-Id: Ibfe0caa9751d3bcc11bfc6e0654a3d1ac35ac8ae
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/src/widgets/widgets/qscrollbar.h b/src/widgets/widgets/qscrollbar.h
index 4af5fb4a55..d044ec0519 100644
--- a/src/widgets/widgets/qscrollbar.h
+++ b/src/widgets/widgets/qscrollbar.h
@@ -78,6 +78,13 @@ private:
 
     Q_DISABLE_COPY(QScrollBar)
     Q_DECLARE_PRIVATE(QScrollBar)
+#ifndef QT_NO_ITEMVIEWS
+    friend class QTableView;
+    friend class QTreeViewPrivate;
+    friend class QCommonListViewBase;
+    friend class QListModeViewBase;
+    friend class QAbstractItemView;
+#endif
 };
 
 #endif // QT_NO_SCROLLBAR