From f688bcd1f14175f1624e2abcc6452882448467c0 Mon Sep 17 00:00:00 2001
From: "Martin T. H. Sandsmark" <martin.sandsmark@kde.org>
Date: Wed, 13 Jul 2016 00:29:15 +0200
Subject: [PATCH] Fix scrolling on hidpi screens

Scrolling with libinput was unbearably slow.

QScrollBar is much better at scrolling than us, so let it handle it.

REVIEW: 128432
BUG: 357618
---
 src/kitemviews/kitemlistcontainer.cpp         | 22 +---------------
 .../private/kitemlistsmoothscroller.cpp       | 25 ++++---------------
 .../private/kitemlistsmoothscroller.h         | 12 ++++-----
 3 files changed, 11 insertions(+), 48 deletions(-)

diff --git a/src/kitemviews/kitemlistcontainer.cpp b/src/kitemviews/kitemlistcontainer.cpp
index 69a0a5f279..6974ebb483 100644
--- a/src/kitemviews/kitemlistcontainer.cpp
+++ b/src/kitemviews/kitemlistcontainer.cpp
@@ -185,27 +185,7 @@ void KItemListContainer::wheelEvent(QWheelEvent* event)
     KItemListSmoothScroller* smoothScroller = scrollHorizontally ?
                                               m_horizontalSmoothScroller : m_verticalSmoothScroller;
 
-    const QScrollBar* scrollBar = smoothScroller->scrollBar();
-    if (!event->pixelDelta().isNull()) {
-        const int numPixels =  event->pixelDelta().y();
-        if (event->modifiers().testFlag(Qt::ShiftModifier)) {
-            const int scrollingDirection = numPixels > 0 ? 1 : -1;
-            smoothScroller->scrollTo(scrollBar->value() - scrollBar->pageStep() * scrollingDirection);
-        } else {
-            smoothScroller->scrollTo(scrollBar->value() - numPixels);
-        }
-    } else {
-        const int numDegrees = event->angleDelta().y() / 8;
-        const int numSteps = qApp->wheelScrollLines() * numDegrees / 15;
-        if (event->modifiers().testFlag(Qt::ShiftModifier)) {
-            const int scrollingDirection = numSteps > 0 ? 1 : -1;
-            smoothScroller->scrollTo(scrollBar->value() - scrollBar->pageStep() * scrollingDirection);
-        } else {
-            smoothScroller->scrollTo(scrollBar->value() - numSteps * scrollBar->pageStep() / 12);
-        }
-    }
-
-    event->accept();
+    smoothScroller->handleWheelEvent(event);
 }
 
 void KItemListContainer::slotScrollOrientationChanged(Qt::Orientation current, Qt::Orientation previous)
diff --git a/src/kitemviews/private/kitemlistsmoothscroller.cpp b/src/kitemviews/private/kitemlistsmoothscroller.cpp
index c89d3cf8f8..6bfdba4c9a 100644
--- a/src/kitemviews/private/kitemlistsmoothscroller.cpp
+++ b/src/kitemviews/private/kitemlistsmoothscroller.cpp
@@ -172,8 +172,7 @@ bool KItemListSmoothScroller::eventFilter(QObject* obj, QEvent* event)
         break;
 
     case QEvent::Wheel:
-        handleWheelEvent(static_cast<QWheelEvent*>(event));
-        return true; // eat event so that QScrollBar does not scroll one step more by itself
+        return false; // we're the ones sending them
 
     default:
         break;
@@ -196,25 +195,11 @@ void KItemListSmoothScroller::handleWheelEvent(QWheelEvent* event)
     const bool previous = m_smoothScrolling;
 
     m_smoothScrolling = true;
-    int numPixels;
-    if (!event->pixelDelta().isNull()) {
-        numPixels = event->pixelDelta().y();
-    } else {
-        const int numDegrees = event->angleDelta().y() / 8;
-        const int numSteps = qApp->wheelScrollLines() * numDegrees / 15;
-        numPixels = numSteps * m_scrollBar->pageStep() / 12;
-    }
-    int value = m_scrollBar->value();
-    if (event->modifiers().testFlag(Qt::ShiftModifier)) {
-        const int scrollingDirection = numPixels > 0 ? 1 : -1;
-        value -= m_scrollBar->pageStep() * scrollingDirection;
-    } else {
-        value -= numPixels;
-    }
-    m_scrollBar->setValue(value);
+
+    QWheelEvent copy = *event;
+    QApplication::sendEvent(m_scrollBar, &copy);
+    event->setAccepted(copy.isAccepted());
 
     m_smoothScrolling = previous;
-
-    event->accept();
 }
 
diff --git a/src/kitemviews/private/kitemlistsmoothscroller.h b/src/kitemviews/private/kitemlistsmoothscroller.h
index fca4b0c53b..a3576a3fdd 100644
--- a/src/kitemviews/private/kitemlistsmoothscroller.h
+++ b/src/kitemviews/private/kitemlistsmoothscroller.h
@@ -77,6 +77,11 @@ public:
     // stable.
     bool requestScrollBarUpdate(int newMaximum);
 
+    /**
+     * Forwards wheel events to the scrollbar, ensuring smooth and proper scrolling
+     */
+    void handleWheelEvent(QWheelEvent* event);
+
 protected:
     virtual bool eventFilter(QObject* obj, QEvent* event);
 
@@ -84,13 +89,6 @@ private slots:
     void slotAnimationStateChanged(QAbstractAnimation::State newState,
                                    QAbstractAnimation::State oldState);
 
-private:
-    /**
-     * Results into a smooth-scrolling of the target dependent on the direction
-     * of the wheel event.
-     */
-    void handleWheelEvent(QWheelEvent* event);
-
 private:
     bool m_scrollBarPressed;
     bool m_smoothScrolling;