PDFViewer: Enable panning with middle mouse button

Enable the use of the middle mouse button for panning in PDFViewer.

Userland/Applications/PDFViewer/PDFViewer.cpp

@@ -113,6 +113,29 @@ void PDFViewer::mousewheel_event(GUI::MouseEvent& event)
     }
 }
 
+void PDFViewer::mousedown_event(GUI::MouseEvent& event)
+{
+    if (event.button() == GUI::MouseButton::Middle) {
+        m_pan_starting_position = to_content_position(event.position());
+        set_override_cursor(Gfx::StandardCursor::Drag);
+    }
+}
+
+void PDFViewer::mouseup_event(GUI::MouseEvent&)
+{
+    set_override_cursor(Gfx::StandardCursor::None);
+}
+
+void PDFViewer::mousemove_event(GUI::MouseEvent& event)
+{
+    if (event.buttons() & GUI::MouseButton::Middle) {
+        auto delta = to_content_position(event.position()) - m_pan_starting_position;
+        horizontal_scrollbar().decrease_slider_by(delta.x());
+        vertical_scrollbar().decrease_slider_by(delta.y());
+        update();
+    }
+}
+
 void PDFViewer::timer_event(Core::TimerEvent&)
 {
     // Clear the bitmap vector of all pages except the current page

Userland/Applications/PDFViewer/PDFViewer.h

@@ -58,6 +58,9 @@ protected:
 
     virtual void paint_event(GUI::PaintEvent&) override;
     virtual void mousewheel_event(GUI::MouseEvent&) override;
+    virtual void mousedown_event(GUI::MouseEvent&) override;
+    virtual void mouseup_event(GUI::MouseEvent&) override;
+    virtual void mousemove_event(GUI::MouseEvent&) override;
     virtual void timer_event(Core::TimerEvent&) override;
 
 private:
@@ -69,4 +72,6 @@ private:
     Vector<HashMap<u32, RefPtr<Gfx::Bitmap>>> m_rendered_page_list;
 
     u8 m_zoom_level { initial_zoom_level };
+
+    Gfx::IntPoint m_pan_starting_position;
 };