Add option to disable smooth scrolling

Even though the animated smooth scrolling transitions are now affected
by the global animation speed setting in Plasma, this is not totally
satisfactory for the full pool of people who hate smooth scrolling, as
it does not address the cases of people using Okular outside of Plasma
or people who generally want animations but just not in Okular's
scrolling implementation.

Accordingly, there is now a GUI option to disable smooth scrolling in
Okular's settings window.

BUG: 420755
FIXED-IN: 20.12

conf/dlggeneralbase.ui

@@ -64,6 +64,13 @@
         <property name="bottomMargin">
          <number>0</number>
         </property>
+        <item>
+         <widget class="QCheckBox" name="kcfg_SmoothScrolling">
+          <property name="text">
+           <string>Use smooth scrolling</string>
+          </property>
+         </widget>
+        </item>
         <item>
          <widget class="QCheckBox" name="kcfg_ShowScrollBars">
           <property name="text">

conf/okular.kcfg

@@ -286,6 +286,9 @@
   <entry key="EditToolBarPlacement" type="Int" >
    <default>0</default>
   </entry>
+  <entry key="SmoothScrolling" type="Bool" >
+   <default>true</default>
+  </entry>
   <entry key="ShowScrollBars" type="Bool" >
    <default>true</default>
   </entry>

doc/index.docbook

@@ -2482,6 +2482,12 @@ Context menu actions like Rename Bookmarks etc.)
 		<sect1 id="configgeneral">
 			<title>General</title>
 			<variablelist>
+                <varlistentry>
+					<term><guilabel>Use smooth scrolling</guilabel></term>
+					<listitem>
+						<para>Whether to display animated transitions when scrolling using the mouse wheel and keyboard.</para>
+					</listitem>
+				</varlistentry>
 				<varlistentry>
 					<term><guilabel>Show scrollbars</guilabel></term>
 					<listitem>

ui/pageview.cpp

@@ -451,7 +451,6 @@ PageView::PageView(QWidget *parent, Okular::Document *document)
             PageView::updateSmoothScrollAnimationSpeed();
         }
     });
-    PageView::updateSmoothScrollAnimationSpeed();
 
     // connect the padding of the viewport to pixmaps requests
     connect(horizontalScrollBar(), &QAbstractSlider::valueChanged, this, &PageView::slotRequestVisiblePixmaps);
@@ -871,6 +870,9 @@ void PageView::displayMessage(const QString &message, const QString &details, Pa
 
 void PageView::reparseConfig()
 {
+    // set smooth scrolling policies
+    PageView::updateSmoothScrollAnimationSpeed();
+
     // set the scroll bars policies
     Qt::ScrollBarPolicy scrollBarMode = Okular::Settings::showScrollBars() ? Qt::ScrollBarAsNeeded : Qt::ScrollBarAlwaysOff;
     if (horizontalScrollBarPolicy() != scrollBarMode) {
@@ -4188,6 +4190,16 @@ void PageView::addSearchWithinDocumentAction(QMenu *menu, const QString &searchT
 
 void PageView::updateSmoothScrollAnimationSpeed()
 {
+    // If it's turned off in Okular's own settings, don't bother to look at the
+    // global settings
+    if (!Okular::Settings::smoothScrolling()) {
+        d->currentShortScrollDuration = 0;
+        d->currentLongScrollDuration = 0;
+        return;
+    }
+
+    // If we are using smooth scrolling, scale the speed of the animated
+    // transitions according to the global animation speed setting
     KConfigGroup kdeglobalsConfig = KConfigGroup(KSharedConfig::openConfig(), QStringLiteral("KDE"));
     const qreal globalAnimationScale = qMax(0.0, kdeglobalsConfig.readEntry("AnimationDurationFactor", 1.0));
     d->currentShortScrollDuration = d->baseShortScrollDuration * globalAnimationScale;