QuickShow: When zooming with the mouse wheel, keep image centered at cursor.

Applications/QuickShow/QSWidget.cpp

@@ -72,12 +72,19 @@ void QSWidget::mousemove_event(GMouseEvent& event)
 void QSWidget::mousewheel_event(GMouseEvent& event)
 {
     auto old_scale = m_scale;
+    auto old_scale_factor = (float)m_scale / 100.0f;
+    auto zoom_point = event.position().translated(-m_bitmap_rect.location());
+    zoom_point.set_x((float)zoom_point.x() / old_scale_factor);
+    zoom_point.set_y((float)zoom_point.y() / old_scale_factor);
     m_scale += -event.wheel_delta() * 10;
     if (m_scale < 10)
         m_scale = 10;
     if (m_scale > 1000)
         m_scale = 1000;
     relayout();
+    auto new_scale_factor = (float)m_scale / 100.0f;
+    auto scale_factor_change = new_scale_factor - old_scale_factor;
+    m_bitmap_rect.move_by(-Point((float)zoom_point.x() * scale_factor_change, (float)zoom_point.y() * scale_factor_change));
     if (old_scale != m_scale) {
         if (on_scale_change)
             on_scale_change(m_scale);

Applications/QuickShow/main.cpp

@@ -56,7 +56,7 @@ int main(int argc, char** argv)
         window->set_title(String::format("QuickShow: %s %s %d%%", path, bitmap->size().to_string().characters(), scale));
     };
 
-    window->set_double_buffering_enabled(false);
+    window->set_double_buffering_enabled(true);
     update_window_title(100);
     window->set_rect(200, 200, bitmap->width(), bitmap->height());