Option for internal PDFium-based viewer on Qt 5.13+

Summary:
Qt 5.13+ has a built-in PDFium-based PDF viewer enabled in
QWebEngineSettings by default. There is no way to disable it at
run time by the user, and leaving it "as is" changes the browser's
behavior, depending on which version of Qt is installed on a
target system. The built-in plugin currently has quite a limited
functionality and, e.g., misses ability to download the document
being viewed.

This adds a checkbox in Preferences->Browsing->Web configuration
to enable/disable usage of internal Qt's PDF viewer on systems
with Qt 5.13 and above.

The feature uses Pepper plugin API (PPAPI) of QtWebEngine. The
corresponding checkbox's text has been renamed to better reflect
its impact on QtWebEngine's operation ("Allow Pepper Plugins
(Flash plugin)" -> "Enable Pepper plugin API (PPAPI)").

To clarify dependency of internal PDF viewer on PPAPI, the
checkbox gets disabled when the user unchecks "Enable Pepper
plugin API (PPAPI)" checkbox. Also, the dependency is mentioned
in its text.

Reviewers: #falkon, SGOrava

Reviewed By: #falkon, SGOrava

Subscribers: pshaw, SGOrava, falkon, alukichev

Tags: #falkon

Differential Revision: https://phabricator.kde.org/D26753

src/lib/app/mainapplication.cpp

@@ -974,6 +974,10 @@ void MainApplication::loadSettings()
     webSettings->setAttribute(QWebEngineSettings::DnsPrefetchEnabled, settings.value(QSL("DNSPrefetch"), true).toBool());
 #endif
 
+#if QTWEBENGINEWIDGETS_VERSION >= QT_VERSION_CHECK(5, 13, 0)
+    webSettings->setAttribute(QWebEngineSettings::PdfViewerEnabled, settings.value(QSL("intPDFViewer"), false).toBool());
+#endif
+
     webSettings->setDefaultTextEncoding(settings.value(QSL("DefaultEncoding"), webSettings->defaultTextEncoding()).toString());
 
     setWheelScrollLines(settings.value(QSL("wheelScrollLines"), wheelScrollLines()).toInt());

src/lib/preferences/preferences.cpp

@@ -121,6 +121,10 @@ Preferences::Preferences(BrowserWindow* window)
     ui->dnsPrefetch->setVisible(false);
 #endif
 
+#if QTWEBENGINEWIDGETS_VERSION < QT_VERSION_CHECK(5, 13, 0)
+    ui->intPDFViewer->setVisible(false);
+#endif
+
     auto setCategoryIcon = [this](int index, const QIcon &icon) {
         ui->listWidget->item(index)->setIcon(QIcon(icon.pixmap(32)));
     };
@@ -284,6 +288,7 @@ Preferences::Preferences(BrowserWindow* window)
     // BROWSING
     settings.beginGroup("Web-Browser-Settings");
     ui->allowPlugins->setChecked(settings.value("allowPlugins", true).toBool());
+    connect(ui->allowPlugins, &QAbstractButton::toggled, this, &Preferences::allowPluginsToggled);
     ui->allowJavaScript->setChecked(settings.value("allowJavaScript", true).toBool());
     ui->linksInFocusChain->setChecked(settings.value("IncludeLinkInFocusChain", false).toBool());
     ui->spatialNavigation->setChecked(settings.value("SpatialNavigation", false).toBool());
@@ -295,6 +300,8 @@ Preferences::Preferences(BrowserWindow* window)
     ui->disableVideoAutoPlay->setChecked(settings.value("DisableVideoAutoPlay", false).toBool());
     ui->webRTCPublicIpOnly->setChecked(settings.value("WebRTCPublicIpOnly", true).toBool());
     ui->dnsPrefetch->setChecked(settings.value("DNSPrefetch", true).toBool());
+    ui->intPDFViewer->setChecked(settings.value("intPDFViewer", false).toBool());
+    ui->intPDFViewer->setEnabled(ui->allowPlugins->isChecked());
 
     const auto levels = WebView::zoomLevels();
     for (int level : levels) {
@@ -539,6 +546,11 @@ Preferences::Preferences(BrowserWindow* window)
     QzTools::setWmClass("Preferences", this);
 }
 
+void Preferences::allowPluginsToggled(bool checked)
+{
+    ui->intPDFViewer->setEnabled(checked);
+}
+
 void Preferences::chooseExternalDownloadManager()
 {
     QString path = QzTools::getOpenFileName("Preferences-ExternalDownloadManager", this, tr("Choose executable location..."), QDir::homePath());
@@ -959,6 +971,7 @@ void Preferences::saveSettings()
     settings.setValue("DisableVideoAutoPlay", ui->disableVideoAutoPlay->isChecked());
     settings.setValue("WebRTCPublicIpOnly", ui->webRTCPublicIpOnly->isChecked());
     settings.setValue("DNSPrefetch", ui->dnsPrefetch->isChecked());
+    settings.setValue("intPDFViewer", ui->intPDFViewer->isChecked());
 
 #ifdef Q_OS_WIN
     settings.setValue("CheckDefaultBrowser", ui->checkDefaultBrowser->isChecked());

src/lib/preferences/preferences.h

@@ -74,6 +74,7 @@ private Q_SLOTS:
     void setManualProxyConfigurationEnabled(bool state);
     void useExternalDownManagerChanged(bool state);
     void changeCachePathClicked();
+    void allowPluginsToggled(bool checked);
 
     void newTabChanged(int value);
     void afterLaunchChanged(int value);

src/lib/preferences/preferences.ui

@@ -1008,7 +1008,14 @@
                  <item>
                   <widget class="QCheckBox" name="allowPlugins">
                    <property name="text">
-                    <string>Allow Pepper Plugins (Flash plugin)</string>
+                    <string>Enable Pepper plugin API (PPAPI)</string>
+                   </property>
+                  </widget>
+                 </item>
+                 <item>
+                  <widget class="QCheckBox" name="intPDFViewer">
+                   <property name="text">
+                    <string>Use built-in PDF viewer (requires PPAPI)</string>
                    </property>
                   </widget>
                  </item>